MAVLink: updated severity values to conform to RFC-5424

Closes #901 

Here's how I changed severities:

was|is
----------|----------
SEVERITY_LOW | MAV_SEVERITY_WARNING
SEVERITY_MEDIUM | MAV_SEVERITY_ALERT
SEVERITY_HIGH | MAV_SEVERITY_CRITICAL
SEVERITY_USER_RESPONSE | MAV_SEVERITY_NOTICE

I also removed the old enum which is no longer needed, and changed some methods which wanted a 'gcs_severity' to want an 'int'.

Tested on a copter only (that's all I have).  But build_all.sh returns no errors.