build: Make custom build system usable

TODO
 - support um arch
 - support incremential builds
 - add targets for generating ready to use images
 - better dependency support (e.g. forcing rebuild after changes in
   settings.yml)

Future possible improvements:
 - distributed builds
 - more robust build_config.yml files
 - caching

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>
build: Remove cmake files

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>
build: Return proper status code

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>