fw landing: improve slope altitude calc to avoid climbout after waypoint. Throttle cut is now defined via altitude

Navigator: add MissionFeasibilityChecker class; performs validation of landing waypoint set-up for fixed wing for now, but can be extended for other checks (e.g. check mission against geofence)

Add an option to the systemreset() call and to the reboot command (-b) to reboot into the bootloader.

The system will remain in the bootloader until it's reset, or until an upload is completed.