linux: fix dependency on mkimage

After the rework of the U-Boot configuration/compilation process, we
need to slightly rework how target/linux/Makefile.in.advanced depends
on mkimage on the host to produce an uImage.

target/linux/Makefile.in doesn't need to be fixed as it just doesn't
handle this dependency for the moment.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>