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

* Move the Makefile up one directory in preparation for supporting
  headers_install.
* 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.
* Update BUILD.bazel to support these changes as well.

Signed-off-by: Will McVicker <willmcvicker@google.com>
Change-Id: I48db5257fb317d2403ce8c3c581efc5353a0b092