If image is always built same way then it is considered 'infra'.
BINARY_SOURCE_IMAGES variable lists toplevel binary/source images.
There is a switch "--infra-rename" to enable renaming of infrastructure
images. Disabled by default to keep compatibility (will be enabled in
future patch).
Implements: blueprint infra-images
Change-Id: Ib5235e3576ef0eae3efab9559ef44227c928067c