firmware-utils mktplinkfw: print amount of exceeding bytes

This is very useful for trimming images towards 4 MiB flash size.

Signed-off-by: Stephan Maka <stephan@spaceboyz.net>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46469 3c298f89-4303-0410-b956-a3cf2f4a3e73