Plane: add multiple takeoff and landing capability without the use of a companion computer

#### Issue details
As discussed with @tridge in [this forum post](https://discuss.ardupilot.org/t/anyone-has-experience-with-long-flights-in-unfamiliar-terrain/22410/8), flying a mission with multiple takeoffs and landings requires something like a companion computer to run the rest of the mission after landing for the first time if no direct telemetry control is present (i.e. re-arm the aircraft after landing, switch to next waypoint).

Removing the need for a companion computer in this situation will be helpful for an increasingly common use case now: autonomous delivery vehicles that operate BVLOS and out of telemetry range and need to
* Land at a remote site and upon cargo loading/unloading and on a button press, take off and RTL
* Or conduct another leg of the mission to another area, doing multiple deliveries in on a single battery/fuel load

Also as discussed, this could be done with an addition to `AP_Button` such as `BTN_ACTION1` or `BTN_ACTION2`, of which one option could be `1: Arm and switch to next mission item`.

I currently would like this functionality on a cargo delivery aircraft being built that is running Ardupilot, and since it will run in a rugged/rural environment, I would prefer having fewer components on board to service to make the entire system more reliable

#### Version
N/A

#### Platform
[ ] All
[ ] AntennaTracker
[X] Copter
[X] Plane
[X] Rover
[X] Submarine

#### Airframe type
X-Frame quadplane currently, but this is more widely applicable

#### Hardware type
Pixhawk 2.1 (Cube), ditto

#### Logs
N/A