- support all combinations of:
    * new root partition is already mounted as /
    * new root partition lives elsewhere
    * new /boot partition is the same as the already configured one
    * new /boot is already mounted
    * new /boot device doesn't exist
 - error out if anything is still mounted that shouldn't be mounted
 - only mount when necessary
 - only use chroot if new root device is not /
 - use traps to always unmount everything and clean tmpfiles upon
   failure
 - consult /etc/fstab of new root for location of old /boot
 - copy over contents of old /boot to new one if necessary