Driver/LX: Make LXNAV Nano detection sticky if identified via port

The is_nano flag was permanently lost if a link timeout occurs.
Introduce a sticky "port_is_nano" flag which survives in this case.
Driver/LX: Also detect LXNAV Nano 3

Relax the Bluetooth name prefix matching. The Bluetooth name of
a Nano 3 starts with "LXNAV-NANO3", e.g. "LXNAV-NANO3-3035".

See #3295. This probably will not fix the declaration but a Nano 3
should be detected properly with this commit.
Engine/Airspace/AirspaceActivity: Assume active if weekday is not known

This is a conservative assumption. Otherwise no airspace warning at
all is generated if the weekday is not known. Discovered because the
Condor simulator does not send a date in the $GPRMC NMEA sentence.