[TASK] Clean up bwl interface

[arnout]

Description

The current BWL interface has evolved historically into its current shape. Because of that, it is a bit more complicated than strictly necessary for prplMesh purposes. In addition, there are probably a few functions that are not really needed.

Several new bwl backends will be added for different chipsets. Therefore, the BWL interface will become external API that is going to be more difficult to change. Thus, we should freeze that API. It is therefore important to do any refactoring now, before more backends are added.

Look at BWL as a whole and determine which API is really needed, and refactor it in that direction.

This ties in with #448 and #435.