Refactor versioning

* Replace the 1-to-1 variable-to-buildtype stuff with a single
  environment variable. CM_[TYPE]=true becomes CM_BUILDTYPE=[TYPE]
* Change the placement of the extra version tag: 10.2-21234567-TYPE-model-TAG
  becomes 10.2-21234567-TYPE-TAG-model, for consistency with the rest.
  The last component of the version should always be the model.
* Add support for the SNAPSHOT build type, for use with the monthlies
* Unknown types will still fallback to UNOFFICIAL
* Accept the jenkins RELEASE_TYPE variable as an alternative to
  CM_BUILDTYPE for compatibility with older branches

Change-Id: Idf96c7ca887747a5ae49a17cf5adf642fb1d561f
Merge remote-tracking branch 'upstream/cm-10.2' into dark-jelly-cm-10.2