redefine format of easyconfig section markers to e.g. [>= 1.2.3], [goolf < 2.0]

changed:
* intuitive order of operator and reference version
* single-space separator

This should make https://github.com/hpcugent/easybuild-framework/pull/693 ready for merging, so further work on fat easyconfigs can be done in follow-up PRs.