cramfs: mark as obsolete

Who needs cramfs when you have squashfs?  At least, we should warn people
that cramfs is obsolete.

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>