Generate the build matrix from the variant files (#3870)

variants/rak4631/platformio.ini

@@ -2,6 +2,7 @@
 [env:rak4631]
 extends = nrf52840_base
 board = wiscore_rak4631
+board_check = true
 build_flags = ${nrf52840_base.build_flags} -Ivariants/rak4631 -D RAK_4631
   -L "${platformio.libdeps_dir}/${this.__env__}/BSEC2 Software Library/src/cortex-m4/fpv4-sp-d16-hard"
   -DGPS_POWER_TOGGLE ; comment this line to disable triple press function on the user button to turn off gps entirely.
@@ -17,4 +18,4 @@ lib_deps =
   rakwireless/RAKwireless NCP5623 RGB LED library@^1.0.2
 debug_tool = jlink
 ; If not set we will default to uploading over serial (first it forces bootloader entry by talking 1200bps to cdcacm)
-;upload_protocol = jlink
+;upload_protocol = jlink