vendor: support custom unofficial build tags

By defining TARGET_UNOFFICIAL_BUILD_ID in a device's cm.mk, users can
build custom CM builds with their own identifier in place.
This adheres to CM's policy that unofficial builds must be
marked as such, as both the filename and internal version will still
contain the word UNOFFICIAL.

Example:
TARGET_UNOFFICIAL_BUILD_ID := CatEater01
results in
cm-11-20131211-UNOFFICIAL-CatEater01-device(.zip)

Change-Id: I61acdf4698a7fe2b35d3d5315be4b444b1b97987