Bugfixes upstream

[dusan19]

Minor bugfixes:

• do not require both python 2 and python 3 to upload the firmware. On my pc without calling python3 explicitly, python 2 was assumed in some places.
• Depending on whether Basic ID 2 was enabled, transmission rate for BT4 was wrong in some cases.
• For BT4, header length was not set initially so the advertiser was crashing when no messages were received, due to length being 0.

[tridge]

looks good, thanks!