Fix and clean up examples/pigweed-app/efr32 build

[mspang]

• Remove all of the duplicate include_dirs that were needed due
  to missing dependencies & improper use of private deps.
• Add back an retargetserial.c file which is needed for this build. It
  was removed in fc59bd2 ("[3rd-party] update openthread and
  ot-br-posix ([3rd-party] update openthread and ot-br-posix #5062)")
• Remove some unusual whitespace in defines

[mspang]

@jepenven-silabs How do we resolve the duplicate symbol error:

obj/third_party/connectedhomeip/third_party/openthread/repo/examples/platforms/efr32/src/libopenthread-efr32.uart.c.o: in function `USART0_RX_IRQHandler':
/__w/connectedhomeip/connectedhomeip/out/lock_app_debug/BRD4161A/../../../examples/lock-app/efr32/third_party/connectedhomeip/third_party/openthread/repo/examples/platforms/efr32/src/uart.c:259: multiple definition of `USART0_RX_IRQHandler'; obj/third_party/connectedhomeip/third_party/efr32_sdk/repo/hardware/kit/common/drivers/sdk.retargetserial.c.o:/__w/connectedhomeip/connectedhomeip/out/lock_app_debug/BRD4161A/../../../examples/lock-app/efr32/third_party/connectedhomeip/third_party/efr32_sdk/repo/hardware/kit/common/drivers/retargetserial.c:120: first defined here

[jmartinez-silabs]

@mspang I did some work to fix pigweed for efr32 after the Openthread submodule update brought some issues with the duplicate symbol error:. It is still on a branch as I wanted to make it better. retargetserial use the same IRQ that is used in the uart implementation in OT for the cli.

This is the file that is built with OT subrepo that brings this conflict

third_party/openthread/platforms/efr32/BUILD.gn
   "${openthread_root}/examples/platforms/efr32/src/uart.c",

It is also problematic for pigweed generally because OT calls efr32UartProcess periodicly and will get the data from the serial instead of the Pigweed functionnality if run on lighting example for example.

So I removed that file "${openthread_root}/examples/platforms/efr32/src/uart.c",

And wanted to reimplement those function in our side and make sure they don't do anything when the example is built for pigweed.

extern "C" __WEAK otError otPlatUartEnable(void)
{
    return OT_ERROR_NOT_IMPLEMENTED;
}

extern "C" __WEAK otError otPlatUartSend(const uint8_t *aBuf, uint16_t aBufLength)
{
    return OT_ERROR_NOT_IMPLEMENTED;
}

extern "C" __WEAK otError otPlatUartFlush(void)
{
    return OT_ERROR_NOT_IMPLEMENTED;
}

extern "C" __WEAK otError otPlatUartDisable(void)
{
    return OT_ERROR_NOT_IMPLEMENTED;
}

extern "C" __WEAK void efr32UartProcess(void)
{
    // 
}

I was also reworking the UART driver to not use retargetserial and use dma