network: Move DHCP config to overlays

...and also enable it on clearfog which previously was not being built
with any networking enabled.

Change-Id: I541f2ecb8828e5bf217e12800024c728e38c6114
diff --git a/board/czechlight/clearfog/overlay/usr/lib/systemd/network/eth0.network b/board/czechlight/clearfog/overlay/usr/lib/systemd/network/eth0.network
new file mode 100644
index 0000000..16ac288
--- /dev/null
+++ b/board/czechlight/clearfog/overlay/usr/lib/systemd/network/eth0.network
@@ -0,0 +1,4 @@
+[Match]
+Name=eth0
+[Network]
+DHCP=yes