Makefile: Have binary_size_check report only first match of _image_binary_end

Makefile: Have binary_size_check report only first match of _image_binary_end

If we have ASSERT macros that validate the position of
_image_binary_end, our awk expression will report a string that causes
the rest of our check to fail with garbage values. Have it exit after
the first match to fix this.

Signed-off-by: Tom Rini <trini@konsulko.com>