Build system cleanup and maintenance

Parent issue for the tor-browser-build cleanup+maintenance work we agreed to prioritise in Gothenburg. Such work would include:

• de-templatising build scripts in favor of scripts which take arguments (so such scripts can be used on their own outside of tor-browser-build)
• cleaning up unused projects (e.g. namecoin and friends, whatever else)
• creating single-source of truth for common params in the firefox and geckoview projects
• removing unused legacy rbm targets, params
• etc

Please link any relevant new+existing issues to this issue so we can track.

/cc @brizental @boklm @pierov