release 1.3.6
rc: expose set_override for use by other modules

Rather than sending rc_override msgs straight from droneapi goo I'd
rather just share this code (so they don't develop disagreements about
desired rc state)
raise version
moved battery handling to its own module
added ARSENL startup script and logging support for sorties
elevation: return None on bad elevation
cope with None return from elevation module
terrain: first version of terrain module
terrain: enable terrain module by default
HIL: fixed module name
terrain: added terrain checking and reporting
raise version
wp: support terrain alt missions
console: show terrain status
raise version
use 'terrainalt' setting for creating mission items and guided mode
wp: fixed wp move of terrain waypoints
raise version
console: show vehicle AGL in console
raise version