On-device installer: pmbootstrap install --ondev

Overview: https://wiki.postmarketos.org/wiki/On-device_installer

Together with pmaports!1296 (merged) and postmarketos-ondev.git, this gets the infrastructure in place to combine pmbootstrap with Calamares. Find the details in the commit messages.

Testplan:

$ cd ~/code/pmaports
$ mrhlpr checkout 1296
$ pmbootstrap zap
$ yes | pmbootstrap install --ondev
$ pmbootstrap qemu

Closes pmaports#463 (closed) (follow-up issues are here)