Add initial support for the on-device installer in pmbootstrap. Let
pmbootstrap create a regular split image, then prepare a new installer
rootfs and copy the previously generated rootfs image into the installer
rootfs. Put the installer rootfs into a new image, with reserved space.
There is more to do from here, such as disabling the generation of the
user account when using --ondev. But this requires support in
postmarketos-ondev first, so let's build that iteratively.
Related: https://wiki.postmarketos.org/wiki/On-device_installer
Related: https://gitlab.com/postmarketOS/postmarketos-ondev/-/issues
ddb5d9ae2c