Add configuration option to boot an alternative 2nd stage #961

Job	Run time
f41 x64 build compile_commands.json	3m 2s
f41 x64 build	2m 13s
f40 x64 build	2m 10s
f41 aa64 cross-build	2m 13s
f40 aa64 cross-build	2m 4s
f41 arm cross-build	2m 5s
f40 arm cross-build	1m 56s
f39 arm cross-build	2m 0s
f39 x64 build	2m 12s
centos9 x64 build	1m 10s
f41 x64 cross-build	2m 2s
centos8 x64 build	1m 13s
f40 x64 cross-build	2m 24s
f39 ia32 build	2m 5s
f39 x64 cross-build	2m 5s
centos8 ia32 build	1m 14s
f41 ia32 cross-build	2m 12s
f40 ia32 cross-build	2m 3s
f39 ia32 cross-build	2m 5s
	38m 28s

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