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

@@ -27,6 +27,10 @@
   #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

@@ -43,6 +43,9 @@
   #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

@@ -47,6 +47,9 @@
 
 #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

@@ -36,6 +36,9 @@
 
 #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