* Change `pmbootstrap flasher flash_system` command to
`pmbootstrap flasher flash_rootfs`
* The old command still works, but all references have been changed to
the new command
* Remove obsolete `pmbootstrap flasher export` (that was changed to
`pmbootstrap export` a few months ago)
* Update `README.md` and ZSH auto completion
* Change the description of the generated rootfs image (not talking
about a system image anymore, mention that it has subpartitions)
* Better description of `pmbootstrap flasher flash_rootfs --partition`
* Allow to specify a custom username in "pmbootstrap init"
* Build chroots have "pmos" instead of "user" as username now
* Installation user UID is 1000 now (as in all other Linux distributions)
* Adjust autologins
* postmarketos-base: enable wheel group for sudo, removed previous sudoers file
* Implement safe upgrade path:
We save the version of the work folder format now, in $WORK/version.
When this file does not exist, it defaults to 0.
In case it does not match the currently required version
(pmb.config.work_version), then ask the user if it should
automatically be upgraded.
* moved export code to pmb/export and split it up
* added deprecation notice to "pmbootstrap flasher export"
* made "pmbootstrap export" work
* adjusted the "pmbootstrap flasher export" hints in the code