cmsdk: Add constraints support for CMHardware features

 * This patch will make it easy to hide preferences based on the
   lack of a CMHardware feature. In the preference, specify this:

     cm:requiresFeature="cmhardware:FEATURE_KEY_DISABLE"

   ..and the preference will magically be removed if the feature
   is not supported.

Change-Id: I51699df7b39bb38b9ea69c93ade658736f89b988