[assembly] bootfs_or_base is available in AIB's and BIB's

While the assembly libraries define PackageSet::BootfsOrBase, it is only
available in the frontends for driver components. Add a bootfs_or_base
package set option to the AIB and BIB interfaces and update the
responsible tools and scripts to ensure they handle this package type
correctly.

Change-Id: Ib2ad49b9cf38c675069b25595a3a196efe642bee
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1547158
Reviewed-by: Aidan Wolter <awolter@google.com>
Fuchsia-Auto-Submit: Gary Bressler <geb@google.com>
Reviewed-by: Aaron Wood <aaronwood@google.com>
Commit-Queue: Gary Bressler <geb@google.com>