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

* mesh::RTCClock class moved to MeshCore.h

variants/xiao_c3/platformio.ini

@@ -3,6 +3,7 @@ extends = esp32_base
 board = seeed_xiao_esp32c3
 build_flags =
   ${esp32_base.build_flags}
+  -I variants/xiao_c3
   -D LORA_TX_BOOST_PIN=D3
   -D P_LORA_TX_LED=D5
   -D PIN_VBAT_READ=D0
@@ -15,6 +16,8 @@ build_flags =
   -D SX126X_DIO2_AS_RF_SWITCH=true
   -D SX126X_DIO3_TCXO_VOLTAGE=1.8
   -D SX126X_CURRENT_LIMIT=130.0f  ; for best TX power!
+build_src_filter = ${esp32_base.build_src_filter}
+  +<../variants/xiao_c3>
 
 [env:Xiao_C3_Repeater_sx1262]
 extends = Xiao_esp32_C3