merge branch 'pr-755'
merge branch 'pr-755'

  Makefile: variable cleanup and creation

LGTMs: da39a3ee5e6b4b0d3255bfef95601890afd80709@cyphar
Closes #755

opencontainers/image-spec,opencontainers/image-spec,jonboulle/image-spec,jonboulle/image-spec