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`.

buildroot/share/PlatformIO/scripts/download_mks_assets.py

@@ -1,5 +1,6 @@
 #
-# download_mks_assets.py
+# buildroot/share/PlatformIO/scripts/download_mks_assets.py
+# Added by HAS_TFT_LVGL_UI to download assets from Makerbase repo
 #
 Import("env")
 import os,requests,zipfile,tempfile,shutil