fs: Kconfig: Add a separate option for FS_CRAMFS

Rather than using CMD_CRAMFS for both the filesystem and its command, we
should have a separate option for each. This allows us to enable CRAMFS
support without the command, if desired, which reduces U-Boot's size
slightly.

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: imply FS_CRAMFS for keymile]
Signed-off-by: Tom Rini <trini@konsulko.com>