kbuild: Clean up the build files

* Update build files now that gs/kernel/device-modules is an external
  module.
* Move the build objects to Kbuild and update Makefile to just compile
  the module.
* Update the trace header defines
* Fix GIT_REPO_TAG to work with kleaf (when compile outside of a git
  tree). Also, use THIS_MODULE->scmversion when available.

Signed-off-by: Will McVicker <willmcvicker@google.com>
Change-Id: Iea1a3b60a30a1144b8416fc5a111b90e122c6a7e