* 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`