In particular:
- .base is now generated from pieces (see image.in/Makefile)
- s/DISTROS/IMAGES/g; s/CONFIGS/DISTROS/g (for clarity)
- s/DISK_LISTS/MAIN_LISTS/g ("disk" was early m-p-d legacy)
- introduced BASE_PACKAGES to complement BASE_LISTS
- minor tweaks to Makefile (ARCH and DATE moved elsewhere)
- libdistro.mk: dropped overlooked IMAGE_INIT_LIST copy
- clean.mk: silly cleanup