🔨 Clean up build scripts (#22264)

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

ini/stm32f4.ini

@@ -112,7 +112,6 @@ build_flags          = ${common_stm32.build_flags} -DHAL_SD_MODULE_ENABLED -DHAL
 board                = marlin_STM32F407VGT6_CCM
 board_build.core     = stm32
 board_build.variant  = MARLIN_F4x7Vx
-board_build.ldscript = ldscript.ld
 board_build.encrypt  = firmware.srec
 # Just openblt.py (not stm32_bootloader.py) generates the file
 board_build.offset   = 0x10000
@@ -222,7 +221,6 @@ extends              = common_stm32
 board                = marlin_STM32F407VGT6_CCM
 board_build.core     = stm32
 board_build.variant  = MARLIN_F4x7Vx
-board_build.ldscript = ldscript.ld
 board_build.offset   = 0x8000
 board_upload.offset_address = 0x08008000
 extra_scripts     = ${common.extra_scripts}
@@ -344,7 +342,6 @@ upload_protocol      = dfu
 monitor_speed        = 500000
 board_build.core     = stm32
 board_build.variant  = MARLIN_F446VE
-board_build.ldscript = ldscript.ld
 board_build.offset   = 0x0000
 extra_scripts        = ${common.extra_scripts}
   pre:buildroot/share/PlatformIO/scripts/generic_create_variant.py
@@ -361,7 +358,6 @@ build_flags          = ${stm_flash_drive.build_flags}
 board                = genericSTM32F407VET6
 board_build.core     = stm32
 board_build.variant  = MARLIN_F4x7Vx
-board_build.ldscript = ldscript.ld
 board_build.offset   = 0x0000
 board_upload.offset_address = 0x08000000
 build_unflags        = ${common_stm32.build_unflags} -DUSBCON -DUSBD_USE_CDC
@@ -387,7 +383,6 @@ build_flags          = ${common_stm32.build_flags} ${stm32f4_I2C1.build_flags} -
 board                = marlin_STM32F407VGT6_CCM
 board_build.core     = stm32
 board_build.variant  = MARLIN_F4x7Vx
-board_build.ldscript = ldscript.ld
 board_build.rename   = Robin_nano_v3.bin
 board_build.offset   = 0xC000
 board_upload.offset_address = 0x0800C000