build: Allow packaging of factory image files into target files

Files may be required inthe target files package for factory flashing
processes. Allow these to be added using PRODUCT_FACTORYIMAGE_FILES.

Change-Id: I74b42daee7b73f762f3c75904c6c278b1e5f5c53
Ticket: RM-112
(cherry picked from commit 68d62a7f)