Makefile: run CONFIG_BOARD_SIZE_LIMIT against .img

With the current Makefile, CONFIG_BOARD_SIZE_LIMIT is used to check
the U-Boot binary without devicetree only. This produces wrong results
when OF_SEPARATE is used.

To fix this, run the CONFIG_BOARD_SIZE_LIMIT check on all .img binaries
as well.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>