Cherry-pick changes required to build for custom boards (#37702)

Author: arun-silabs

examples/platform/silabs/uart.cpp

@@ -41,7 +41,9 @@ extern "C" {
 
 #if SLI_SI91X_MCU_INTERFACE
 #include "USART.h"
+#if defined(SL_SI91X_BOARD_INIT)
 #include "rsi_board.h"
+#endif // SL_SI91X_BOARD_INIT
 #include "rsi_debug.h"
 #include "rsi_rom_egpio.h"
 #else // For EFR32

src/platform/silabs/platformAbstraction/WiseMcuSpam.cpp

@@ -33,7 +33,9 @@
 // TODO add includes ?
 extern "C" {
 #include "em_core.h"
+#if defined(SL_SI91X_BOARD_INIT)
 #include "rsi_board.h"
+#endif // SL_SI91X_BOARD_INIT
 #include "sl_event_handler.h"
 
 #ifdef SL_CATALOG_SIMPLE_BUTTON_PRESENT
@@ -119,7 +121,9 @@ CHIP_ERROR SilabsPlatform::Init(void)
 void SilabsPlatform::InitLed(void)
 {
     // TODO
+#if defined(SL_SI91X_BOARD_INIT)
     RSI_Board_Init();
+#endif // SL_SI91X_BOARD_INIT
     SilabsPlatformAbstractionBase::InitLed();
 }