vim: install vi symlink

Install a 'vi' symlink to win over busybox vi (more features) and in
case busybox isn't around, for people expecting plain simple 'vi' to
call the editor.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>