[MTD] mtdoops: Add a magic number to logged kernel oops

Add a magic number to logged kernel oops messages so that they
can be more accurately detected rather than just having to rely
on the sequence number. This also allows easier detection of
saved crashes by userspace.

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>