ar71xx: Add new subtarget nand for WNDR4300

ar71xx: Add new subtarget nand for WNDR4300

Add new subtarget nand with UBI/UBIFS including GLUEBI and squashfs.
This target will be used for NAND based router which make use of
UBI (and maybe UBIFS later on) in order to provide proper NAND
support.

Signed-off-by: Stefan Agner <stefan@agner.ch>
[juhosg:
  - tweak target.mk:
     change BOARDNAME variable,
     improve Target/Description,
     remove targz and ubifs from FEATURES and add broken flag,
  - optimize kernel configuration:
     disable ubifs, it is not used yet,
     disable unused MTD options,
     disable SPI support
     disable unused network drivers,
     tune kernel command line,
  - remove image/ubinize.cfg, it will be in a follow-up patch
  - move profile modifications into a separate patch]
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38646 3c298f89-4303-0410-b956-a3cf2f4a3e73