Looks as though I've already got avahi installed
Results if attempting to update avahi on my onion:

root@firstfloorpowder:~# opkg update
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/base/Packages.gz.
Updated list of available packages in /var/opkg-lists/chaos_calmer_base.
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/base/Packages.sig.
Signature check passed.
Downloading http://repo.onion.io/omega/packages/Packages.gz.
Updated list of available packages in /var/opkg-lists/chaos_calmer_onion.
Downloading http://repo.onion.io/omega/packages/Packages.sig.
Signature check passed.
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/packages/Packages.gz.
Updated list of available packages in /var/opkg-lists/chaos_calmer_packages.
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/packages/Packages.sig.
Signature check passed.
root@firstfloorpowder:~# opkg install avahi-daemon-service-ssh avahi-daemon-serv
ice-http
Package avahi-daemon-service-ssh (0.6.31-12) installed in root is up to date.
Package avahi-daemon-service-http (0.6.31-12) installed in root is up to date.