* BIG refactor: 'board' and 'radio' objects now defined in 'target.h/.cpp'

* mesh::RTCClock class moved to MeshCore.h

variants/rak4631/platformio.ini

@@ -4,6 +4,7 @@ platform = https://github.com/maxgerhardt/platform-nordicnrf52.git#rak
 board = wiscore_rak4631
 board_check = true
 build_flags = ${nrf52840_base.build_flags}
+  -I variants/rak4631
   -D RAK_4631
   -D PIN_USER_BTN=9
   -D RADIO_CLASS=CustomSX1262
@@ -13,6 +14,7 @@ build_flags = ${nrf52840_base.build_flags}
   -D SX126X_RX_BOOSTED_GAIN=1
 build_src_filter = ${nrf52840_base.build_src_filter}
   +<helpers/nrf52/*.cpp>
+  +<../variants/rak4631>
 lib_deps =
   ${nrf52840_base.lib_deps}
   adafruit/Adafruit SSD1306 @ ^2.5.13