Added command "fstypes" to list supported/included filesystems.

Signed-off-by: Niel Fourie <lusus@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
[trini: Limit to sandbox]
Signed-off-by: Tom Rini <trini@konsulko.com>