fs: Kconfig: Add a separate config for FS_CBFS

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

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: imply FS_CBFS on SYS_COREBOOT]
Signed-off-by: Tom Rini <trini@konsulko.com>