Fix strange RTL behavior

This commits have fixed #1441 for me. Please review and cross test.

The behaviour should now be:

For COM_DL_LOSS_EN ==0:

- Mission finished, no RC, datalink ok: AUTO_RCRECOVER (RTL)
- Mission finished, no RC, datalink lost: AUTO_RCRECOVER (RTL)

For COM_DL_LOSS_EN == 1:

- Mission finished, no RC, datalink ok: AUTO_MISSION (loitering)
- Mission finished, no RC, datalink lost: AUTO_RCRECOVER (RTL)

Hmm, I see your point.

As I see it this datalink enabled is more like an additional control that allows us to get the UAV back if it is stuck somewhere after the mission is finished and we've lost RC.
However, I don't think that it should trigger RTGS whenever the datalink is lost. It probably depends on the use case: e. g. usual flying vs. something like the OBC. Or it boils down to the question if we should allow a UAV to continue it's mission if it looses RC and the datalink, so basically every control.

My proposition would be:

### During the mission:

Datalink enabled:
- Datalink ok, RC ok: MISSION
- Datalink ok, RC lost: MISSION
- Datalink lost, RC ok: **MISSION (or RTGS?)**
- Datalink lost, RC lost: **RTGS**

Datalink disabled:
- Datalink ok, RC ok: MISSION
- Datalink ok, RC lost: MISSION
- Datalink lost, RC ok: MISSION
- Datalink lost, RC lost: **RCRECOVER (or MISSION?)**

### And after the mission is finished:

Datalink enabled:
- Datalink ok, RC ok: MISSION (loitering)
- Datalink ok, RC lost: **MISSION (loitering)**
- Datalink lost, RC ok: **MISSION (loitering), (or RTGS?)**
- Datalink lost, RC lost: **RTGS**

Datalink disabled:
- Datalink ok, RC ok: MISSION (loitering)
- Datalink ok, RC lost: **RCRECOVER**
- Datalink lost, RC ok: MISSION (loitering)
- Datalink lost, RC lost: **RCRECOVER**