CRAMFS: support cramfs in RAM

cramfsls and cramfsload are added to the command list.
A cramfs placed at 'cramfs_addr' can the be listed with 'cramfsls' and files
can be loaded with 'cramfsload'. 'cramfs_addr' is an environment variable
specifying the address the cramfs is located.
This works for powerpc and for ARM.

Use CONFIG_CMD_CRAMFS.

Signed-off-by: Andreas Huber <andreas.huber@keymile.com>
Signed-off-by: Heiko Schocher <hs@denx.de>