pmbootstrap aportgen binutils: fix builddir and pass apkbuild-lint (aportgen channels: part 1.5)

Standalone binutils related patches from the channels-aportgen branch, see #1917 (closed). This does not depend on !1939 (merged).

Also this does not yet make pmbootstrap aportgen binutils suitable for multiple channels, there are follow-up patches in this branch (that are still WIP), that will make this possible.

I'm creating a separate MR now, because binutils needs to be updated in pmaports, and it needs to be done with the builddir fix from this merge request.

Find the details in the commit messages.