uboot-mkimage: new package

The mkimage for the host is now built by a package, so that
boot/u-boot/u-boot.mk is no longer responsible for doing this. It
makes things a lot easier to handle, as linux/linux.mk now simply
depends on host-uboot-mkimage.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>