km/common: enable cramfs and cramfs cmdline

All the km boards uses CRAMFS images where the kernel is stored.
This isn't architecture specific because we use it on ARM and
POWERPC.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>