DRONE-15 - buildflag

deactivate Drone with Feature flag
improve Gradle build Feature output
add LegoNXT build flag
fix typos

Incoming testrun: https://jenkins.catrob.at/view/All-Categories/view/Catroid-multi-job/job/Catroid-Multi-Job-Custom-Branch/1659

@wagnergerald 
You can use "gradle assembleDebug installDebug -Pparrot_ar_drone_enabled=true" for building or just change the flag in build.gradle and use "git update-index --assume-anchanged build.gradle". Could you please check if this works in the IDE also for e.g. running a single test? Normal building is possible with a Gradle Run configuration and an additional "Script parameter".