Make PRODUCT_CHARACTERISTICS a single-valued variable

Values should be comma-separated, and things won't go down well
if there are multiple words in this variable.

Bug: 123783710
Test: Put this in product_config.mk and ran multiproduct_kati:
      ifneq (1,$(words $(PRODUCT_CHARACTERISTICS)))
        $(error oops)
      endif
Change-Id: I5627f416093d68d8d19916a914739686b74d392f