Create the device rootfs with a single invocation of apk

We currently require a whole bunch of steps to build a rootfs (and this only became more complicated with /usr merge...).

We should be able to create the rootfs with a single invocation of apk static on the host, apk has its own chroot feature for running install scripts. This should speed up the install process (in large part by not running apk through qemu) and make it easier for folks to build their own pmOS images without pmbootstrap.

This requires:

• having apk.static support /usr merge (which requires getting https://gitlab.alpinelinux.org/alpine/apk-tools/-/merge_requests/155 re-done and merged).
• Support doing user configuration on first boot #2337
• automate timezone setup to happen after connecting to the internet for the first time.

... and probably more things I can't remember otomh.