Factorize the u-boot images code

This patch introduces the BR2_LINUX_KERNEL_UBOOT_IMAGE boolean to
factorize more code that will be shared in the next patches that
introduces other uImage-like targets.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>