Adding defines for EMERGENCY_STOP_EVENT and CLEAR_EMERGENCY_STOP_EVENT.
Adding defines for EMERGENCY_STOP_EVENT and CLEAR_EMERGENCY_STOP_EVENT.

These are defined in the well-known events spec but are not defined
currently in the defs, I've added these and a short description (may
need edits). These will primarily be used by a command station that
supports LCC.

bakerstu/openmrn,bakerstu/openmrn,bakerstu/openmrn,bakerstu/openmrn,bakerstu/openmrn,bakerstu/openmrn