avahi: unset FILES_avahi to avoid packaging files there

avahi: unset FILES_avahi to avoid packaging files there

The main avahi recipe doesn't package anything into ${PN}.  If a library is
enabled but not put into a separate package it ends up in ${PN} and all the
packages are renamed using it's soname - ie libdns-sd-daemon.

Avoid this by clearing FILES_avahi so extra files are unpackaged instead of
creating a new package.  Also actually remove /var as the comments claim so the
avahi package is truly empty.

(From OE-Core rev: 324c6ac33ab44e4c1949b8001a53a8ffd2a979e2)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>