Add "make image=memcached"

Add images/memcached.py, so that "make image=memcached" would work.

To use this, you'll also need to check out a recent version of the
osv-apps repository (in apps directory, check out the master branch,
and git pull).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>