Building mruby is done with `make clean all -j$(nproc)`, but build fails
randomly because `clean` and `all` run in parallel, and `clean` may be
executed after `all` starts.
Therefore, change it as follows.
* Remove `clean` because it seems to be unneeded in the first place.
* Use `rake` command because mruby uses Rake for builds (`Makefile` is a
limited wrapper for calling `rake`).
* Use `-m` option to run builds in parallel.
* Remove `-j` option because build time seems to be almost the same even
if `-j` option is specified.
* [presubmit] Enforce language attribute in projectt.yaml to be always set.
* Update documentation, better presubmit check, new project template.
* add docstring to templates.py
* Add example values in the project.yaml template and remove python value for now
* Add "project: c++" to 256 projects
* format
* Add labels and selective_unpack sections to the presubmit check
* fix incorrect auto_ccs format in three projects
* fix nss emails after rebase
DavidKorczynski