Makefile: reusable function for BOARD_SIZE_CHECK

Makefile: reusable function for BOARD_SIZE_CHECK

Carve out function size_check from macro BOARD_SIZE_CHECK. This will allow
us to reuse the function for other file size checks.

Depending on the value of CONFIG_BOARD_SIZE_LIMIT an error like the
following is thrown:

u-boot-dtb.img exceeds file size limit:
  limit:  409516 bytes
  actual: 444346 bytes
  excess: 34830 bytes
make: *** [Makefile:1212: u-boot-dtb.img] Error 1

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>