u-boot: convert to GENTARGETS

The feature of building mkimage for the target is removed, it wasn't
building with current U-Boot, and doesn't make a lot of sense (mkimage
is needed only to prepare a kernel image, or an U-Boot script).

The feature of building mkimage for the host is moved to a proper
package. It duplicates a few things (U-Boot URL, etc.) but it makes
things a lot cleaner than having it handled by boot/u-boot/u-boot.mk.

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