dhcpcd: Include ntp.conf hook

Although `dhcpcd` includes a hook for configuring the NTP daemon from
DHCP options, it is not installed by default.  To enable it, one must
pass `-with-hook=ntp.conf` to the `configure` script before compiling
and installing the package.  Buildroot does not pass this flag by
default, but it does allow us to add it to the list of flags to pass in
our external configuration.

external.mk

@@ -1 +1,3 @@
 include $(sort $(wildcard $(BR2_EXTERNAL_metricspi_PATH)/package/*/*.mk))
+
+DHCPCD_CONFIG_OPTS += -with-hook=ntp.conf