dnsmasq: don't use network functions at boottime (FS#1542)

dnsmasq: don't use network functions at boottime (FS#1542)

As dnsmasq is started earlier than netifd usage of network.sh functions
at boottime will fail; therefore don't call at boottime the functions
which construct the dhcp pool/relay info.
As interface triggers are installed the dhcp pool/relay info will be
constructed when the interface gets reported as up by netifd.
At the same time also register interface triggers based on DHCP relay
config.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
(cherry picked from commit 2336b942b37f265c59547d738ca558b61102833d)