Clean up Makefile
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 *'.