Fixed a bug where non-auto hold in the debug console induced 100% CPU load even when the widget was hidden

The MAVLINK_MESSAGE_LENGTHS macro can be used to detect mavlink
messages that don't have the expected length. It is done as a macro so
that projects that don't want to pay the cost of a 256 byte overhead
can avoid the check

Set to 38400,8,N,1 just need to map the baud rate enums now.

qextserial library is no longer maintained, suggest we move
to this library, also greater chance for thread safety,
library now builds in cmake, will add Serial2 sources to
handle the new qserial, should easily plug into serial interface
now.