Streamlined USB flashing

Streamlined USB flashing, For more details see https://github.com/bitcraze/crazyflie-firmware/pull/939

assigned to @crglick

added 17 commits

• 5476234c...278583d3 - 16 commits from branch master
• b88835af - Merge branch 'master' into 'dfu_flash'

Compare with previous version

enabled an automatic merge when the pipeline for b88835af succeeds

merged

mentioned in commit af9bf373