how can i get out of vi editor when esc won't work?

What you want to is to ignore changes on tracked files. This cannot achieved (as Charles Bailey says) correctly, neither with .gitignore nor with .git/info/exclude.

You'll need to use git update-index:

git update-index --assume-unchanged build/conf/a.conf
git update-index --assume-unchanged build/conf/b.conf

will achieve what you want: the files are always assumed unchanged.

If you want to track changes in these files again, use --no-assume-unchanged.

Finally, if you want to know which files are currently in the --assume-unchanged mode, ask git for

git ls-files -v | grep -e "^[hsmrck]"