pmbootstrap aportgen musl-* etc: support release channels

When running pmbootstrap aportgen musl-armhf, and the v20.05 branch is checked out in pmaports.git, then pmbootstrap should:

• read from the pmaports.cfg that the current channel is stable
• read from channels.cfg in master branch: branch_aports=3.11-stable, mirrordir_alpine=v3.11
• use that information to generate the new musl-armhf package - don't grab it from edge, but from v3.11

Same for all other alpine-based packages generated with pmbootstrap aportgen.