Create the image mappings BDMs earlier in the boot

Refactor the processing of BDMs defined in the image (either as a block
device mapping in the properties or mappings of blank devices) and move
it to the same method where the rest of BDMs are processed.

Partial-bug: #1297325
Change-Id: Ida0ddb6efcb0014890762df000eaea679f9b1d7b
Merge "Create the image mappings BDMs earlier in the boot"