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.

The packet_count was actually an error_count, therefore we should
process RC input only when the error_count since the last packet is 0.

Also, this commit fixes the RSSI scaling for st24.

The firmware binary is to large to fit into the onboard memory of the Parrot
Bebop. It could be uploaded to the emmc, but for ease of use it would be nice
to have it in /usr/bin. To strip the binary seems to be the best option right now.

Fixes,
error: ignoring return value of ‘ssize_t write(int, const void*,
size_t)’, declared with attribute warn_unused_result
[-Werror=unused-result]

The pthread linker flags were changed in c6a26415 and it was questioned in #5504 if the
exception for the Bebop is necessary. It is not, so remove those lines from the cmake
file.

Signed-off-by: Roman <bapstroman@gmail.com>

Signed-off-by: Roman <bapstroman@gmail.com>