avahi: use systemd.bbclass for avahi-systemd

The original behaviour is to enable the service in the package postinst phase. This patch
will enable the service also when building the rootfs, so it fixes the case when no
package-management is enabled. It also now delegates systemd enablement to the systemd
bbclass, instead of using explicit code in the recipe itself.

Signed-off-by: Sander van Grieken <sander@outrightsolutions.nl>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>