GitLab

Prepare the Makefile for producing more than one image, and fix small
issues along the way.

- Rename 'IMAGE' to 'IMAGES', and generate COMPRESSED from IMAGES (the
  '.xz' suffix is appended to every filename in IMAGE).
- Fix the image being rebuilt every time even when no change occurred by
  making the 'local.sh-link' prerequisite order-only.
- Make CHECKSUM depend on COMPRESSED so that the checksum file is
  updated if the images changed.
- Use variable STAMPFILE instead of repeating 'tmp/$(IMAGE).stamp'.
- Remove all .tmp file in the 'clean' target. This also eliminates a
  potentially dangerous 'rm -rf *'.