For Travis CI - skip APK test run when packaging.
General build script cleanup:
- Constrain build option values in cmake-gui.
- Constrain build configuration for both multi-configuration and single-configuration generators.
- Mark cached variables as advanced or internal as appropriate.
Enhance Rakefile to autoload other *.rake scripts in ".rake" directory.
Also create symlink to Rakefile when scaffolding.
[ci skip]
Merge branch 'cleaning-up-build-scripts-for-android' into master.
Suppress CMake unused variable warning for Android platform.
For Travis CI - simplify the ci_rebase rake task.
On normal commit, use the origin/master. On git tagging commit, use the 'tag' branch.
Also when on git taggig commit, prevent API doc update on the master branch because it is already too late to do so as the commit won't be in the tag anyway.
For Travis CI - switch CI build to use Raspberry-Pi build environment.