🔨 Clean up build scripts (#22264)

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

buildroot/share/PlatformIO/scripts/marlin.py

@@ -67,11 +67,3 @@ def encrypt_mks(source, target, env, new_name):
 
 def add_post_action(action):
 	env.AddPostAction(join("$BUILD_DIR", "${PROGNAME}.bin"), action);
-
-# Apply customizations for a MKS Robin
-def prepare_robin(address, ldname, fwname):
-	def encrypt(source, target, env):
-		encrypt_mks(source, target, env, fwname)
-	relocate_firmware(address)
-	custom_ld_script(ldname)
-	add_post_action(encrypt);