For unibuilds (at least, for some), the bundled firmware may now
consist of multiple images, one for each model.  For those builds,
the metadata.json still reports one version as "the" firmware version
for the board, even though no such thing exists.

As a specific example, in json metadata for a recent coral build,
you see these things:
    Python 2.7.6 (default, Nov 23 2017, 15:49:48)
    [GCC 4.8.4] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import json
    >>> metadata = json.load(open('metadata.json'))
    >>> metadata['board-metadata']['coral']['main-firmware-version']
    u'Google_Coral.10068.37.0'
    >>> metadata['board-metadata']['coral']['models']['nasher']['main-readonly-firmware-version']
    u'Google_Coral.10068.34.0'

The 'main-firmware-version' entry for board 'coral' is meaningless,
and should be omitted.

I note that if we make this change, it should mitigate some of
the ongoing hassles associated with  bug 782272 .