Co-authored-by: Scott Lahteine <github@thinkyhead.com>

* Add 10K to marlin_blackSTM32F407VET6 (typo?)
* Document custom build scripts.
* Add a Robin common build script.
* Extraneous .ldscript specifiers

* 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`.