generate_extra_images: Create $(PRODUCT_OUT)/system/etc

Required to fix a race condition due to a missing dependency on
$(PRODUCT_OUT)/system/etc in build-cdrom-target

Change-Id: I22ce8ee6bdd91fe65b582d6792aed62a07358391
(cherry picked from commit 6880cc7d31ef259fdd23711d52e7403bb209c2c3)