Merge pull request #289 from adam2872/RAK4631-user-button

Implement user button on RAK4631 using analogue pin 31 (same as MT)

variants/rak4631/platformio.ini

@@ -7,6 +7,7 @@ build_flags = ${nrf52840_base.build_flags}
   -I variants/rak4631
   -D RAK_4631
   -D PIN_USER_BTN=9
+  -D PIN_USER_BTN_ANA=31
   -D PIN_BOARD_SCL=14
   -D PIN_BOARD_SDA=13
   -D PIN_OLED_RESET=-1