Default packages are now selected via image build feature.

* Introduce a build feature per image type (i.e. minimum_image, 
  bootstrap_image and regular_image).
* Activate the build feature matching the active image type.
* Use the image build features to group the default packages.
Replace HAIKU_{BOOTSTRAP,MINIMUM}_BUILD with HAIKU_BUILD_TYPE.

* Instead of separate variables, HAIKU_BUILD_TYPE is set to one of
  'bootstrap', 'minimum' or 'regular'.
* Adjust uses of HAIKU_BOOTSTRAP_BUILD accordingly.