distinction between env vars and options is not clear

For both disk-image-create and ramdisk-image-create, there isn't currently a clear distinction between what is (or should be) set via ENV var and what should be set as a command-line option. Furthermore, some settings (eg, IMAGE_NAME vs "-o <name>") can be set either way, which is quite confusing.

Chris Jones proposed:
  If a setting controls the overall build environment,
  it's an env var, but if it controls a specific feature of the build, it's an argument. TMP_DIR is unlikely
  to change if you do 20 builds in a row, but -o is extremely likely to be build-specific

I think this is a reasonable distinction, and some of the older code (eg, lib/img-defaults) needs a bit of cleanup.