udhcpd.conf: Provide DNS server IP for BTSs

If we don't provide a DNS server via DHCP to the BTSs, then their
ntpdate will not succeed in contacting pool.ntp.org (resolver failure),
which in turn means they have the wrong date, which again in turn leads
to failing opkg update or openvpn certificate verification failures.

recipes-apps/sysmocom-udhcpd-config/files/sysmocom-bsc/udhcpd.conf

@@ -6,3 +6,4 @@ interface eth1
 option subnet 10.23.24.0
 option router 10.23.24.1
 option broadcast 10.23.24.255
+option dns 8.8.8.8