Code review comment for lp://staging/~julian-edwards/maas/dup-boot-source-selections-bug-1360280

ISTR Blake saying that we had to coalesce the entries for one OS/release rather than just dropping all-but-one, because the entries may all have different arches, subarches and labels — all of which are perfectly valid choices, but which need to be recorded in a single row, not several.

So, if we don't want to have settings go away surprisingly, (1) is a good option.

That said, could we add a persistent, user-clearable error* after doing (2) that says something like "MAAS's records of your boot source settings may have changed; check the Images tab to ensure that your boot image settings are correct."

*Not sure we actually have the facility to do this; I was thinking of the persistent error stuff, but that all seems to be auto-cleared or not at all.