network: add example file that enables DHCP on ethernet links

The file has instructions how to "enable" it by symlinking into the
appropriate place. If we create a different mechanism to do enablement
later on, we can always adjust the instructions.

Closes #3998.

network/80-ethernet.network.example

@@ -0,0 +1,17 @@
+# SPDX-License-Identifier: CC0-1.0
+#
+# This example config file is installed as part of systemd.
+# It may be freely copied and edited (following the Creative Commons Zero v1.0 Universal License).
+#
+# To use the file, one of the following methods may be used:
+# 1. add a symlink from /etc/systemd/network to the current location of this file,
+# 2. copy the file into /etc/systemd/network or one of the other paths checked
+#    by systemd-networkd and edit it there.
+# This file should not be edited in place, because it'll be overwritten on upgrades.
+
+# Enable DHCPv4 and DHCPv6 on all ethernet links
+[Match]
+Type=ether
+
+[Network]
+DHCP=yes

network/meson.build

@@ -5,6 +5,7 @@ if conf.get('ENABLE_NETWORKD') == 1
                      '80-container-host0.network',
                      '80-container-ve.network',
                      '80-container-vz.network',
+                     '80-ethernet.network.example',
                      '80-vm-vt.network',
                      '80-wifi-adhoc.network',
                      '80-wifi-ap.network.example',