HAOS builds add a lot of files and things get quickly messy. Use a
directory per build.
Also don't abort the complete build if a single board failed, we still
might be interested in the rest.
* Add development build version part to version number
Add third part in the version number to indicate development builds.
Generate a default version number based on the date, e.g.
"5.6.dev20201124".
* Add GitHub Action workflow for development builds
Add another GitHub workflow for development builds. Make it triggered
only for now. The version number is generated by the workflow and
passed to all builds to make sure all builds have the same development
build version.
* Add documentation
