Use mirrordir_pmos and mirrordir_alpine from channels.cfg to generate
the mirror URLs for postmarketOS and Alpine, which get written to
/etc/apk/repositories and which postmarketOS uses to download the
APKINDEX files.
Remove hardcoded "master" at the end of the postmarketOS mirror and use
mirrordir_pmos instead (which is "master" for the edge channel). Let the
postmarketOS mirror end in a '/' for consistency with the Alpine mirror
in pmb/config/__init__.py.
Remove obsolete --alpine-version. To experiment with a different Alpine
version, one should pass a custom --config-channels from now on.
Ask for release channel and switch pmaports branch to the related branch
defined in channels.cfg.
Store in pmbootstrap.cfg whether the user chose a channel (boolean). If
the user did not choose a channel yet, suggest the recommended channel
from channels.cfg (currently "edge").
