create specific firmware online

Sorry for the delay with this...

Espruino doesn't use the 'releases' feature - which I believe is a way to attach binaries to GitHub projects (could be wrong here).

Instead, each release is tagged (which is merely a pointer to a specific Git commit).

In actual fact your quick and ugly solution is basically what I do. If you look in the Makefile you'll see:

git tag | grep RELEASE_ | sort | tail -1

So I get the list of tags, use any starting with RELEASE_, then sort them and take the last one :)