fix build

Add hidl-gen as a dependency on HAL modules, and set the -r path
properly in the absence of $TOP being defined.

Proper fix will include auto-generating the makefile correctly form
hidl-gen.

Change-Id: I9dbe27b80c6c9ab4be9f88bf50496bdfd7cee423
Signed-off-by: Iliyan Malchev <malchev@google.com>