firmware-utils/mktplinkfw: add support for the TL-MR13U board

Signed-off-by: Oleg Titov <oleg.titov@gmail.com>
Patchwork: http://patchwork.openwrt.org/patch/4063/


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

SVN-Revision: 38065