rework reform-boot-config

rework reform-boot-config

• support all combinations of:
  • new root partition is already monted 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 uponfailure
• consult /etc/fstab of new root for location of old /boot
• copy over contents of old /boot to new one if necessary