ar71xx: change title and description for NAND subtarget

The nand subtarget supports Mikrotik boards only. Change
the title and the description to reflect that.

Based on the patch by Stefan Agner:
http://patchwork.openwrt.org/patch/4123/



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

SVN-Revision: 38508