board/common: new ifduown2 dhcp policy

Do not wait for dhclient to acquire first release, this will cause
unnecessary delays when booting, and not provide any real value in our
system.  Because we will never have any service that depends on a
network address being set when starting up, this is a feature that
Finit can provide for us as a condition when starting such services.

Also, never set global "README" in multiple policy files, this will
cause ifupdown2 to crash in horrific ways.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

board/common/rootfs/etc/network/ifupdown2/policy.d/dhcp.json

@@ -0,0 +1,5 @@
+{
+    "dhcp": {
+	"defaults": { "dhcp-wait": "no" }
+    }
+}

board/common/rootfs/etc/network/ifupdown2/policy.d/mgmt.json

@@ -1,5 +1,4 @@
 {
-    "README": "Default mgmt interface prefix is 'eth'",
     "main": {
 	"module_globals": { "mgmt_intf_prefix": "mgmt" }
     }