[nrfconnect] Refactored configuration for child images

[kkasperczyk-no]

Problem

Currently used approach for nrfconnect regarding child images results in significant duplication of configuration files,
what hardens maintenance.

Change overview

• Created Kconfig.mcuboot.defaults file that sets default
  configuration for all examples using mcuboot child image
• Created Kconfig.multiprotocol_rpmsg file that sets
  default configuration for all examples using multiprotocol
  rpmsg child image
• Flatten configuration structure by removing
  configuration/board-name/ files and creating common prj.confs
  in the application root directory
• Removed from rpc.overlay comment describing how to use it,
  as it duplicates example README.md information

Testing

No functional changes, verified manually building.

[github-actions]

PR #17174: Size comparison from 845b38d to d3ffc11

Full report (27 builds for cc13x2_26x2, efr32, esp32, k32w, linux, mbed, nrfconnect, p6, telink)

platform	target	config	section	845b38d	d3ffc11	change	% change
cc13x2_26x2	lock-ftd	LP_CC2652R7	(read only)	674515	674515	0	0.0
			(read/write)	176964	176964	0	0.0
			.bss	81520	81520	0	0.0
			.data	3216	3216	0	0.0
			.rodata	80563	80563	0	0.0
			.text	593472	593472	0	0.0
	lock-mtd	LP_CC2652R7	(read only)	623651	623651	0	0.0
			(read/write)	154320	154320	0	0.0
			.bss	77248	77248	0	0.0
			.data	3216	3216	0	0.0
			.rodata	80451	80451	0	0.0
			.text	542712	542712	0	0.0
	pump-app	LP_CC2652R7	(read only)	681875	681875	0	0.0
			(read/write)	170780	170780	0	0.0
			.bss	81928	81928	0	0.0
			.data	3248	3248	0	0.0
			.rodata	75243	75243	0	0.0
			.text	606148	606148	0	0.0
	pump-controller-app	LP_CC2652R7	(read only)	676219	676219	0	0.0
			(read/write)	176164	176164	0	0.0
			.bss	81656	81656	0	0.0
			.data	3212	3212	0	0.0
			.rodata	79027	79027	0	0.0
			.text	596708	596708	0	0.0
efr32	lighting-app	BRD4161A	(read only)	904908	904908	0	0.0
			(read/write)	133160	133160	0	0.0
			.bss	131120	131120	0	0.0
			.data	2040	2040	0	0.0
			.text	904900	904900	0	0.0
		BRD4161A+rpc	(read only)	932780	932780	0	0.0
			(read/write)	149108	149108	0	0.0
			.bss	146888	146888	0	0.0
			.data	2220	2220	0	0.0
			.text	932772	932772	0	0.0
	window-app	BRD4161A	(read only)	842196	842196	0	0.0
			(read/write)	131228	131228	0	0.0
			.bss	129280	129280	0	0.0
			.data	1948	1948	0	0.0
			.text	842188	842188	0	0.0
esp32	all-clusters-app	c3devkit	(read only)	991900	991900	0	0.0
			(read/write)	1461762	1461762	0	0.0
			.dram0.bss	62936	62936	0	0.0
			.dram0.data	14356	14356	0	0.0
			.flash.rodata	198984	198984	0	0.0
			.flash.text	991900	991900	0	0.0
			.iram0.text	62572	62572	0	0.0
		m5stack	(read only)	1044615	1044615	0	0.0
			(read/write)	462816	462816	0	0.0
			.dram0.bss	68464	68464	0	0.0
			.dram0.data	34160	34160	0	0.0
			.flash.rodata	228048	228048	0	0.0
			.flash.text	1039231	1039231	0	0.0
			.iram0.text	123415	123415	0	0.0
k32w	light	k32w061+release	(read/write)	685148	685148	0	0.0
			.bss	78208	78208	0	0.0
			.data	2036	2036	0	0.0
			.text	599104	599104	0	0.0
	lock	k32w061+release	(read/write)	689932	689932	0	0.0
			.bss	78784	78784	0	0.0
			.data	1996	1996	0	0.0
			.text	603352	603352	0	0.0
linux	chip-tool-no-interactive-ipv6only	arm64	(read only)	10281420	10281420	0	0.0
			(read/write)	492177	492177	0	0.0
			.bss	41009	41009	0	0.0
			.data	1168	1168	0	0.0
			.data.rel.ro	388800	388800	0	0.0
			.dynamic	560	560	0	0.0
			.got	57392	57392	0	0.0
			.init	24	24	0	0.0
			.init_array	184	184	0	0.0
			.rodata	512540	512540	0	0.0
			.text	8665812	8665812	0	0.0
	thermostat-no-ble	arm64	(read only)	2348476	2348476	0	0.0
			(read/write)	150497	150497	0	0.0
			.bss	63025	63025	0	0.0
			.data	1424	1424	0	0.0
			.data.rel.ro	78264	78264	0	0.0
			.dynamic	560	560	0	0.0
			.got	4768	4768	0	0.0
			.init	24	24	0	0.0
			.init_array	368	368	0	0.0
			.rodata	144428	144428	0	0.0
			.text	1976576	1976576	0	0.0
mbed	lock-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2366844	2366844	0	0.0
			.bss	185060	185060	0	0.0
			.data	5840	5840	0	0.0
			.text	1329444	1329444	0	0.0
nrfconnect	all-clusters-app	nrf52840dk_nrf52840	(read/write)	1153375	1153375	0	0.0
			bss	135540	135540	0	0.0
			rodata	145832	145832	0	0.0
			text	793388	793388	0	0.0
	lighting-app	nrf52840dk_nrf52840	(read/write)	1140223	1140223	0	0.0
			bss	139276	139276	0	0.0
			rodata	121232	121232	0	0.0
			text	801024	801024	0	0.0
		nrf52840dk_nrf52840+rpc	(read/write)	1104331	1104331	0	0.0
			bss	139512	139512	0	0.0
			rodata	112356	112356	0	0.0
			text	773444	773444	0	0.0
		nrf52840dongle_nrf52840	(read/write)	1154495	1154495	0	0.0
			bss	140276	140276	0	0.0
			rodata	120812	120812	0	0.0
			text	803900	803900	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	1048886	1048886	0	0.0
			bss	136340	136340	0	0.0
			rodata	114404	114404	0	0.0
			text	716184	716184	0	0.0
	lock-app	nrf52840dk_nrf52840	(read/write)	1040511	1040511	0	0.0
			bss	129220	129220	0	0.0
			rodata	114080	114080	0	0.0
			text	718740	718740	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	949566	949566	0	0.0
			bss	126316	126316	0	0.0
			rodata	107308	107308	0	0.0
			text	634052	634052	0	0.0
	pump-app	nrf52840dk_nrf52840	(read/write)	1059475	1059475	0	0.0
			bss	129136	129136	0	0.0
			rodata	116244	116244	0	0.0
			text	735520	735520	0	0.0
	pump-controller-app	nrf52840dk_nrf52840	(read/write)	1041599	1041599	0	0.0
			bss	128880	128880	0	0.0
			rodata	112416	112416	0	0.0
			text	721756	721756	0	0.0
p6	all-clusters-app	default	(read/write)	2512552	2512552	0	0.0
			.bss	118464	118464	0	0.0
			.data	2768	2768	0	0.0
			.text	1470816	1470816	0	0.0
	light-app	default	(read/write)	2413576	2413576	0	0.0
			.bss	111960	111960	0	0.0
			.data	2576	2576	0	0.0
			.text	1371840	1371840	0	0.0
	lock-app	default	(read/write)	2377208	2377208	0	0.0
			.bss	111704	111704	0	0.0
			.data	2536	2536	0	0.0
			.text	1335472	1335472	0	0.0
telink	lighting-app	tlsr9518adk80d	(read/write)	799240	799240	0	0.0
			bss	70064	70064	0	0.0
			noinit	40416	40416	0	0.0
			text	568274	568274	0	0.0