GCS Config: if no board then have default max rate

In the case of a flaky connection it is possible to connect to
the FC but not get an update of FirmwareIAPObj in which case no
valid board plugin is found. This was causing a crash but we
should handle this more gracefully.
Merge pull request #1045 from peabody124/firmwareiap_missing_crash

GCS Config: if no board then have default max rate