Panic

A new mechanism to handle unrecoverable problems, by either halting the system (if the DEVELHELP macro is defined, as suggested by Ludwig Hortmann) or performing an hard reboot.
A portable function to force system reboot is also provided.
(Note that the ARM implementation is incomplete, as I don't know this (these) architecture(s) enough, and haven't found a generic, portable way to force an hardware reset).