Add AP_Terrain to features that can be included/excluded

[rmackay9]

I was surprised to see that "Terrain" (aka AP_Terrain) is not on our list of available features. I suspect there's a reason for that but in any case, I definitely think users should be able to enable/disable this feature

This is related to this wiki issue and PR:

Issue: ArduPilot/ardupilot_wiki#7199
PR: ArduPilot/ardupilot_wiki#7200