Add AP_Terrain to features that can be included/excluded

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: https://github.com/ArduPilot/ardupilot_wiki/issues/7199
PR: https://github.com/ArduPilot/ardupilot_wiki/pull/7200

Start by reviewing the linked ArduPilot wiki issue #7199 and pull request #7200, then locate the CustomBuild list of features that can be included or excluded. Add AP_Terrain to that list and verify that Terrain can be enabled and disabled through the custom build options.

Written by the indexing model from the issue text.