It seems that there is a concept of a "support" branch in git flow. This is used to add a hotfix to an earlier release.

This thread has more information, with these examples:

git checkout 6.0
git checkout -b support/6.x
git checkout -b hotfix/6.0.1

... make your fix, then:

git checkout support/6.x
git merge hotfix/6.0.1
git branch -d hotfix/6.0.1
git tag 6.0.1

or using git flow commands

git flow support start 6.x 6.0
git flow hotfix start 6.0.1 support/6.x

... make changes then:

git flow hotfix finish 6.0.1