Add -fmerge-all-constants flag

platformio.ini

 [common]
 default_src_filter = +<src/*> -<src/config> -<src/HAL> +<src/HAL/shared>
 extra_scripts = pre:buildroot/share/PlatformIO/scripts/common-cxxflags.py
-build_flags = -fmax-errors=5 -g -D__MARLIN_FIRMWARE__
+build_flags = -fmax-errors=5 -g -D__MARLIN_FIRMWARE__ -fmerge-all-constants
 lib_deps =
   U8glib-HAL=https://github.com/MarlinFirmware/U8glib-HAL/archive/bugfix.zip
   LiquidCrystal@1.3.4
 #
 [env:melzi]
 extends       = env:sanguino_atmega1284p
-build_flags   = ${common.build_flags} -fmerge-all-constants
+build_flags   = ${common.build_flags}
 lib_ignore    = TMCStepper
 upload_speed  = 57600