This change addresses a few nits pointed out in the Gerrit
review (Change-Id: Idf912ff2146434b666fdb4250dc1ecad39bc5a04).
Change-Id: I8668b02a2f46427567526189caa2040fc1e3b402
Story: 1526753
Task: 28098
When ironic builds UEFI-bootable ISO image, it extracts
EFI system partition image (`efiboot.img`) from the
`deploy_iso` ISO image.
This change allows supplying EFI system partition image to
the ISO image building routines in form of a local file or
UUID or URI reference.
The motivation behind this change is to make UEFI-bootable image
building process more efficient and functional. This change is
thought of as a prerequisite for the upcoming Redfish-based
virtual media boot feature.
Story: 1526753
Task: 28098
Change-Id: Idf912ff2146434b666fdb4250dc1ecad39bc5a04
