Raspberry Pi improvements

Some other small improvements:

Raspberry Pi Zero:

• Since there are issues with the GPU driver (see https://wiki.postmarketos.org/wiki/Raspberry_Pi_Zero#Issues) the memory reserved to the GPU can be lowered to the minimal
• Always install the bluetooth firmware as in the RPI3

Raspberry Pi 3:

• Remove the unnecessary kernel subpackage since there is only one choice

TODO:

• Test on Raspberry Pi Zero
• Test on Raspberry Pi 3

(please do not squash!)