Pins/tests followup (#21268)

Missing commit from #21254
Co-authored-by: Scott Lahteine <github@thinkyhead.com>

Marlin/src/pins/lpc1768/pins_BTT_SKR_V1_4.h

   #define BOARD_INFO_NAME "BTT SKR V1.4"
 #endif
 
+#ifndef BOARD_CUSTOM_BUILD_FLAGS
+  #define BOARD_CUSTOM_BUILD_FLAGS -DLPC_PINCFG_UART3_P4_28
+#endif
+
 //
 // SD Connection
 //

Marlin/src/pins/ramps/pins_LONGER3D_LKx_PRO.h

   #warning "Serial 3 is originally reserved to Y limit switches. Hardware changes are required to use it."
 #endif
 
+// Custom flags and defines for the build
+//#define BOARD_CUSTOM_BUILD_FLAGS -D__FOO__
+
 #define BOARD_INFO_NAME "LGT KIT V1.0"
 
 //

Marlin/src/pins/ramps/pins_RAMPS.h

 
 #include "env_validate.h"
 
+// Custom flags and defines for the build
+//#define BOARD_CUSTOM_BUILD_FLAGS -D__FOO__
+
 #ifndef BOARD_INFO_NAME
   #define BOARD_INFO_NAME "RAMPS 1.4"
 #endif

Marlin/src/pins/ramps/pins_RAMPS_S_12.h

 
 #include "env_validate.h"
 
+// Custom flags and defines for the build
+//#define BOARD_CUSTOM_BUILD_FLAGS -D__FOO__
+
 #ifndef BOARD_INFO_NAME
   #define BOARD_INFO_NAME "RAMPS S 1.2"
 #endif

buildroot/share/PlatformIO/scripts/common-dependencies-post.py

+#
+# common-dependencies-post.py
+# Convenience script to add build flags for Marlin Enabled Features
+#
+
+Import("env")
+Import("projenv")
+
+def apply_board_build_flags():
+	if not 'BOARD_CUSTOM_BUILD_FLAGS' in env['MARLIN_FEATURES']:
+		return
+	projenv.Append(CCFLAGS=env['MARLIN_FEATURES']['BOARD_CUSTOM_BUILD_FLAGS'].split())
+
+# We need to add the board build flags in a post script
+# so the platform build script doesn't overwrite the custom CCFLAGS
+apply_board_build_flags()

platformio.ini

   pre:buildroot/share/PlatformIO/scripts/common-dependencies.py
   pre:buildroot/share/PlatformIO/scripts/common-cxxflags.py
   pre:buildroot/share/PlatformIO/scripts/preflight-checks.py
+  post:buildroot/share/PlatformIO/scripts/common-dependencies-post.py
 build_flags        = -fmax-errors=5 -g3 -D__MARLIN_FIRMWARE__ -fmerge-constants
 lib_deps           =
 
 extends  = common_LPC
 board    = nxp_lpc1769
 
-# BTT SKR 1.4 needs a UART3 tweak
-[env:LPC1768_btt_skr_v1_4]
-platform    = ${env:LPC1768.platform}
-extends     = env:LPC1768
-build_flags = ${env:LPC1768.build_flags} -DLPC_PINCFG_UART3_P4_28
-
 #################################
 #                               #
 #      STM32 Architecture       #