[MTD] oops and panic message logging to MTD device

Kernel oops and panic messages are invaluable when debugging crashes.
These messages often don't make it to flash based logging methods (say a
syslog on jffs2) due to the overheads involved in writing to flash.

This patch allows you to turn an MTD partition into a circular log
buffer where kernel oops and panic messages are written to. The messages
are obtained by registering a console driver and checking
oops_in_progress. Erases are performed in advance to maximise the
chances of a saving messages.

To activate it, add console=ttyMTDx to the kernel commandline (where x
is the mtd device number to use).

Signed-off-by: Richard Purdie <rpurdie@openedhand.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>