main/postmarketos-dev: new aport

This package is meant to encompass what an average developer would want from a postmarketOS image. We plan to provide -dev variants of some postmarketOS images for some devices for use by the Linux mobile developer community. These should include useful tools out of the box and provide a familiar shell and coreutils.

@craftyguy note: I didn't have any hand in creating this patch, I just picked it from the master_staging_systemd branch and submitted it here (after talking with @calebccff) separately from that branch. Please direct all reviews/comments to them :D