dnsmasq: Don't add local hostname if ula prefix is not specified

Commit 6a7e56b adds support for adding local hostname for own lan ula adress
but if ula prefix is not specified results into an invalid config (address=/OpenWrt.lan/1)
causing dnsmasq not to start up.
Use lanaddr6 when adding local hostname as the lan ula address is constructed based on the
UCI parameters ip6hint and ip6ifaceid and thus not always ula prefix suffixed with 1

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

SVN-Revision: 48495