Fix aux_config warning

Don't declare targets during config files, as they're loaded in other
contexts as well. In this case, `droidcore` was not defined when we were
handling CleanSpec.mk files, leading to:

build/make/core/aux_config.mk:185: warning: real file "droidcore" depends on PHONY target "auxiliary"

Bug: 123583617
Test: m nothing; check for warnings
Change-Id: I71b8819deb81442810c7dd5c681ddb38a1171cff