ramips: build factory images for Netgear EX2700

ramips: build factory images for Netgear EX2700

This patch adds support for building factory and sysupgrade images
for the Netgear EX2700 that don't require modification of u-boot
environment variables.

The bootloader on this device expects the kernel partition to end
on a 64k block boundary. The last 64 byte of the kernel partition
must contain a valid uImage header - in the stock firmware, this is
the uImage header of the root filesystem. For this patch, we're using
the uImage header of a 0 byte partition (ex2700-fakeroot.uImage).

Signed-off-by: Joseph C. Lehner <joseph.c.lehner@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48771 3c298f89-4303-0410-b956-a3cf2f4a3e73