brunch: add a lazier mode

Since all of CM's targets have a predictable PRODUCT_NAME and BUILD_VARIANT
(i.e., "cyanogen_$(PRODUCT_DEVICE)-eng"), brunch can now be invoked passing
just the PRODUCT_DEVICE ("brunch passion" for the N1 setup+build, for
example).
The "classic" long form of $(PRODUCT_NAME)-$(BUILD_VARIANT) can still be used
if desired ("brunch cyanogen_passion-eng" still works).

Change-Id: I494a7fccdab58efed634418163df553fc1b2e5f1