Flag to reduce Melzi binary size (#14730)

platformio.ini

 platform      = atmelavr
 framework     = arduino
 board         = sanguino_atmega1284p
-build_flags   = ${common.build_flags}
+build_flags   = ${common.build_flags} -fmerge-all-constants
 upload_speed  = 57600
 lib_deps      = ${common.lib_deps}
 lib_ignore    = TMCStepper, TMC26XStepper