Update to support gs/kernel/device-modules as an external module

* Include the Makefile.include from gs/kernel/device-modules which
  updates EXTRA_CFLAGS and EXTRA_SYMBOLS to be able to include headers
  from there as well as link against modules from there.
* Consolidate make targets.
* Update BUILD.bazel to support these changes as well.

Signed-off-by: Will McVicker <willmcvicker@google.com>
Change-Id: Ie48f05b2c30a8cf194ccaf17a5369922c177ae3c