uboot-phytec

kbuild: add misc targets, help, ubootrelease, ubootversion

"help" target has been copied from Linux 3.13 and adjusted
for U-Boot.
Unlike Linux, cleaning is done on 4 levels:
clean, clobber, mrproper, distclean.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>