Rename, clean up boards/variants (#21655)

* Consolidate variant scripts
* Rename Marlin-local boards
* Simplify variants where possible
* Rename variants
* CHITU_F103 and MEEB_3DP: Maple platform `platformio-build-stm32f1.py` uses the 'board' name, not 'board_build.variant' so folder names match 'board' and not `board_build.variant`.

.github/workflows/test-builds.yml

@@ -90,6 +90,7 @@ jobs:
         - NUCLEO_F767ZI
         - REMRAM_V1
         - BTT_SKR_SE_BX
+        - chitu_f103
 
         # Put lengthy tests last