Emmet is a third party plugin and not a part of Sublime Text officially. Its github page is here.
A quick examination of Emmet's source reveals a fix that should give you what you want. In the file emmet-app.js, line 11998 (current version) we have:
'\n<!-- /<%= attr("id", "#") %><%= attr("class", ".") %> -->',
Replace the starting \n with a space to remove the linebreak, and you can modify the rest of the string to taste as well. You'll need to restart Sublime for changes to take effect.
Bear in mind that if you modify that file on your local installation, any update to Emmet via package control will overwrite the file and lose your changes. The solution is to add "Emmet" to the array "auto_upgrade_ignore" in package control user prefs:
If these changes may be of use to you on another Sublime installation, or to other users, consider forking Emmet on github and committing your changes there. That way you'll never lose the changes, can keep track of them and share them with others.