ipmi: Add a panic handler for IPMI users

Users of the IPMI code had their own panic handlers, but the
order was not necessarily right, the base IPMI code would
need to handle the panic first, and the user had no way to
know if the IPMI interface could run at panic time.

Add a panic handler to the user interface, it is called if
non-NULL and the interface the user is on is capable of panic
handling.  It also cleans up the panic log handling a bit to
reuse the existing interface loop in the main panic handler.

Signed-off-by: Corey Minyard <cminyard@mvista.com>