ipq806x: fix fakerootfs position

The fakeroot header is expected by the netgear bootloader in the last
64 bytes of the last block used by the uImage. With the current
linux-4.9 uImage being more than 128k smaller than the linux-4.4 uImage
the bootloader was unable to locate the rootfs ih_magic and was
refusing to load and start the kernel.

Fixes: FS#542

Signed-off-by: Thomas Reifferscheid <thomas@reifferscheid.org>