'append-uImage-fakehdr' can now accept magic number as a second, optional
parameter (passed directly to 'mkimage' command with '-M' option). This
enables construction of proper Netgear-specific fake rootfs images
(required for flashing WNDR4300 for example).
Signed-off-by: Michal Cieslakiewicz <michal.cieslakiewicz@wp.pl>