px4_log: flush each line
px4_log: flush each line

Without this fix all PX4 output is buffered and presented at the very
end of a scripted MAVSDK test which makes the log hard to read.

acfloria/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware