Kbuild: centralize MKIMAGE and cmd_uimage definitions

All ARCHs have the same definition of MKIMAGE. Move it to Makefile.lib
to avoid duplication.

All ARCHs have similar definitions of cmd_uimage. Place a sufficiently
parameterized version in Makefile.lib to avoid duplication.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Mike Frysinger <vapier@gentoo.org> [Blackfin]
Tested-by: Michal Simek <monstr@monstr.eu> [Microblaze]
Tested-by: Guan Xuetao <gxt@mprc.pku.edu.cn> [unicore32]
Signed-off-by: Michal Marek <mmarek@suse.cz>