ramips: add support for Netgear WN3000RPv3

This patch adds support for the Netgear WN3000RPv3
http://www.netgear.com/support/product/wn3000rpv3.aspx

Specifications:
- SoC: MediaTek MT7620A (580MHz, ramips)
- RAM: 32MB DDR
- Storage: 8MB NOR SPI flash
- Wireless: builtin MT7620A, 2x2:2 with u.FL connectors
- Ethernet: 1x100M
- Serial: JP1 header, 57600-8N1
- Stock firmware based on OpenWRT Kamikaze

Like the EX2700, the bootloader expects a secondary image signature,
see https://forum.openwrt.org/viewtopic.php?pid=312577#p312577
This is why the same fakeroot image is used for the WN3000

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>