ARM: mach-shmobile: mackerel: Add zboot support

When CONFIG_ZBOOT_ROM is selected, the resulting zImage file will be small
boot loader and may be burned to rom or flash.

Cc: Magnus Damm <magnus.damm@gmail.com>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>