bluetooth support on pinephone

[norayr]

for bluetooth to work on pinephone we need the following:

• bluetooth firmware.
  it can be taken from https://github.com/ubports/linux-firmware-pine64-rtl8723-bt
  i built the firmware, installed deb file and bt started to be recognized.

• user 'user' should be member of bluetooth group.
  that is i guess necessary for any phone.

i suggest to add the linux-firmware-pine64-rtl8723-bt package as default in pinephone distribution, and modify image builder scripts to add user to the bluetooth group.

[IMbackK]

dupe of #327