move rak4631 specific files into variant folder

variants/rak4631/platformio.ini

@@ -15,7 +15,6 @@ build_flags = ${nrf52840_base.build_flags}
   -D SX126X_CURRENT_LIMIT=140
   -D SX126X_RX_BOOSTED_GAIN=1
 build_src_filter = ${nrf52840_base.build_src_filter}
-  +<helpers/nrf52/RAK4631Board.cpp>
   +<../variants/rak4631>
 lib_deps =
   ${nrf52840_base.lib_deps}

variants/rak4631/target.h

@@ -3,7 +3,7 @@
 #define RADIOLIB_STATIC_ONLY 1
 #include <RadioLib.h>
 #include <helpers/RadioLibWrappers.h>
-#include <helpers/nrf52/RAK4631Board.h>
+#include <RAK4631Board.h>
 #include <helpers/CustomSX1262Wrapper.h>
 #include <helpers/AutoDiscoverRTCClock.h>
 #include <helpers/SensorManager.h>