firmware-utils/mktplinkfw: add an option to generate 16MiB images

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>
Patchwork: http://patchwork.openwrt.org/patch/2449/


Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 40034