Build System: fix autogen_test_config problem

If there happen to be any dependencies added to target of $(test_config),
it'll cause problem in the autogen_config. We fix it by using $< in
the autogen command.

BUG: 109736180
Test: lunch walleye_coverage-userdebug; make adbd_test
Change-Id: I22ab100c5a3f9fa058c1edd66f9e5cfdf5b7f855