pmbootstrap init: Allow switching between mainline and downstream kernel (for devices that have support for both)

Created by: ollieparanoid

@MayeulC wrote in https://github.com/postmarketOS/pmbootstrap/issues/756#issuecomment-368332143:

[...] idea regarding multiple kernels: maybe pmbootstrap will be able to offer the user a choice at some point; I think the way pacman handles multiple providers is nice, and such a mechanism could be unified for choosing between DE, kernel and other components.

While we can't implement that at the package manager level (apk) that easily, we can do it in pmbootstrap init. We could use the same mechanism with subpackages to choose the different kernels like we do from now on with the nonfree packages (#1254).