Add /var/www to device table

The /var/www directory is listed in /etc/passwd in the skeleton target
filesystem as the home directory of the www-data user (uid 33).

In the final target filesystem, /var/www should be owned by www-data. This
is important for the lighttpd package, for example.

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>