setup-alpine: add setup-devd script

The setup-devd script handles the choice between mdev/mdevd/eudev at installation time. It's a major refactor of the way Alpine handles device managers.

So far mdev always had a special, ad-hoc position, due to being bundled with Busybox; it was enabled at installation time, and eudev was kinda sorta working around it by doing various hacks. With the introduction of mdevd, we need to clean this up, give every device manager equality of treatment, and ensure users always know what they're getting. setup-devd explicitly makes users choose between the three options they have.

Merging this does not impact edge directly, so the merge can be performed as soon as this change is deemed safe. The impact on edge will happen when the alpine-conf aport is bumped to 3.14.0; this must be done after a series of commits preparing aports for the new device manager infrastructure (and provided separately as a MR for aports).