Use OUT_DIR_COMMON_BASE if OUT_DIR is unset.

In build/make/core/envsetup.mk the logic for setting OUT_DIR is this:

ifeq (,$(strip $(OUT_DIR)))
ifeq (,$(strip $(OUT_DIR_COMMON_BASE)))
OUT_DIR := $(TOPDIR)out
else
OUT_DIR := $(OUT_DIR_COMMON_BASE)/$(notdir $(PWD))
endif
endif

To get the correct value for its out_dir variable, buildbot-build.sh
needs to do the same, otherwise it'll generate incorrect target names
in environments where OUT_DIR_COMMON_BASE is set but OUT_DIR unset.

Test: art/tools/buildbot-build.sh --host

Change-Id: I558949655a97b47ce3c560ed70310c9768e1da5c