New image type that generates a Boot ISO. The ISO contains a RHEL Edge
commit and an installer. On Boot, it sets up a new RHEL Edge system
with the commit.
The RHEL Edge commit (ostree commit) is downloaded during build from a
URL that should be supplied with the compose request. The commit's hash
and URL need to be added to the Sources list in the Manifest.
Unlike other types, the new image type defines its own "build" package
set that is added to the distro and arch build package lists.
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