fix dhcp on USB Ethernet being unreliable

This seems to have been caused by 2 issues:
- The service file was installed in network.target, but WantedBy was set to
  multi-user.target
- The usb0 interface (probably) was not initialized fully, since SystemD could not ping/arp the gateway

meta-ntb/recipes-connectivity/dhcp/dhcp/dhcpd.service

+[Unit]
+Description=DHCPv4 Server Daemon
+Documentation=man:dhcpd(8) man:dhcpd.conf(5)
+After=network.target
+After=time-sync.target
+
+[Service]
+PIDFile=@localstatedir@/run/dhcpd.pid
+EnvironmentFile=@SYSCONFDIR@/default/dhcp-server
+EnvironmentFile=-@SYSCONFDIR@/sysconfig/dhcp-server
+ExecStartPre=@base_bindir@/touch @localstatedir@/lib/dhcp/dhcpd.leases
+ExecStart=@SBINDIR@/dhcpd -f -cf @SYSCONFDIR@/dhcp/dhcpd.conf -pf @localstatedir@/run/dhcpd.pid $DHCPDARGS -q $INTERFACES
+
+[Install]
+WantedBy=multi-user.target

meta-ntb/recipes-connectivity/dhcp/dhcp_%.bbappend

 FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
 
-
-# for some reason appending to FILES does not work
-# INSANE_SKIP_${PN} += "installed-vs-shipped"
-
-FILES_${PN}-server_append = " \
-  /lib/systemd/system/network.target.wants/dhcpd.service \
-  /etc/systemd/system/network.target.wants \
-  /etc/systemd/system/network.target.wants/dhcpd.service \
-  /lib/systemd/system/dhcpd.service \
-"
-
-do_install_append() {
-  install -d ${D}/etc/systemd/system/network.target.wants
-  install -m 644 ${D}/lib/systemd/system/dhcpd.service ${D}/etc/systemd/system/
-	cd ${D}
-	ln -s etc/systemd/system/dhcpd.service etc/systemd/system/network.target.wants/dhcpd.service 
-}
\ No newline at end of file
+SYSTEMD_AUTO_ENABLE_${PN}-server = "enable"
\ No newline at end of file

meta-ntb/recipes-connectivity/ethernet/usb-net/20-usb.network

@@ -2,6 +2,12 @@
 Name=usb0
 
 [Network]
-Address=192.168.7.2/24
-Gateway=192.168.7.3
-DNS=8.8.8.8
+DNS=0.0.0.0
+
+[Address]
+Address=192.168.7.2
+
+[Route]
+Destination=192.168.7.0/24
+Gateway=192.168.7.2
+