ar71xx: add profile and fixup image generation for the TL-MR10U board

Create a separate board profile and update the profile
name in image/makefile. Also use the correct machtype
string in kernel command line.

Based on andelf's patch:
http://andelf.diandian.com/post/2013-05-22/40050677370

Signed-off-by: Oleg Titov <oleg.titov@gmail.com>
Patchwork: http://patchwork.openwrt.org/patch/3840/
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37550 3c298f89-4303-0410-b956-a3cf2f4a3e73