In unrecoverable situations we should provide a common way to output the
cause of the error and then reset the CPU.  The panic() function is
mean to be exactly that.  It outputs the error-cause, stack-trace, and
firmware revision, accompanied by a link to the issue-tracker to
encourage people to report the error.  After a timeout of ~1.5s it
resets the CPU and reboots.

Future Work:

 - Right now, the stack-trace only has a depth of one which is the
   return address from where panic() was called.  In the future it might
   make sense to provide a deeper stack-trace if a robust implementation
   is possible.
 - Integration of @msgctl's faultscreen (!79) so users who don't have
   the serial console open at all times can also see what happened.

Signed-off-by: Rahix <rahix@rahix.de>