c/fields: Warn on font packages without a M-A field

Warn on font packages without a Multi-Arch field or if its value
is not foreign (or "allowed").

[nthykier: merge two conditions and remove a level of indentation]

Signed-off-by: Vasudev Kamath <kamathvasudev@gmail.com>
Signed-off-by: Niels Thykier <niels@thykier.net>