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

variants/t-echo/platformio.ini

@@ -2,6 +2,7 @@
 [env:t-echo]
 extends = nrf52840_base
 board = t-echo
+board_check = true
 debug_tool = jlink
 
 # add -DCFG_SYSVIEW if you want to use the Segger systemview tool for OS profiling.
@@ -23,4 +24,4 @@ lib_deps =
   ${nrf52840_base.lib_deps}
   https://github.com/meshtastic/GxEPD2#55f618961db45a23eff0233546430f1e5a80f63a
   lewisxhe/PCF8563_Library@^1.0.1
-;upload_protocol = fs
+;upload_protocol = fs