Replace NETGEAR_KERNEL_MAGIC by UIMAGE_MAGIC to better match the
variable's purpose. This allows to drop the custom
Build/netgear-uImage.
Signed-off-by: Sander Vanheule <sander@svanheule.net>
[keep UIMAGE_MAGIC definitions even for default value]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>