This could lead to an exception if serial is not available.

This configures the PWM output for the landing gear.

it generates unnecessary load. If really needed, it can still be enabled
manually.

fixes a 'test: syntax error' message on startup. There were two problems:
- the expansion of $MAVLINK_F lead to multiple arguments in the test
  when the variable contained spaces. Fixed with ""
- the x prevents interpretation as a unary expression, if $MAVLINK_F starts
  with a - character (in that case the expansion would be:
  if [ -r 1200 ... and nsh interprets - as unary expression)

This limits the maximum bandwidth of ulog streaming to 70% of the specified
mavlink datarate. If less is used, the leftover is assigned to the mavlink
streams, if more is used, it starts to drop.

mavlink status outputs the currently used rate, to check if a link is
saturated.

evaluated with: logger start -e -t -m all
and then make sure to get an error printf in the mavlink writer backend,
eg. for an ack timeout.

tested on Pixracer: 14 still produces some dropouts once in a while, but I
think it's a fair tradefoff between RAM usage & dropouts. The queue needs
about 3.5KB of RAM.

When topic sizes/logging rates change, this will have to be reevaluated.

Will be necessary when using multiple backends in parallel.