feat(Makefile): allow setting dracut version via environment variables

[bdrung]

Changes

To ease packaging dracut for Debian/Ubuntu, please support setting the dracut main/full version via environment variables:

$ DRACUT_MAIN_VERSION=007
$ export DRACUT_FULL_VERSION=007-bond
$ make dracut-version.sh
$ grep VERSION dracut-version.sh
DRACUT_VERSION=007-bond

Checklist

• [x] I have tested it locally
• [x] I have reviewed and updated any documentation if relevant
• [ ] I am providing new code and test(s) for it

[aafeijoo-suse]

I am not against this PR, but this PR does not resolve #2169 .

Right.