add settings to enable readonly fs

recipes-core/init-ifupdown/files/interfaces

@@ -0,0 +1,3 @@
+allow-hotplug wlan0
+iface wlan0 inet dhcp
+    wpa-conf /var/etc/network/wpa_supplicant.conf

recipes-core/init-ifupdown/init-ifupdown_%.bbappend

@@ -1,3 +1,9 @@
+FILESEXTRAPATHS:prepend := "${THISDIR}/files:"
+SRC_URI += "file://interfaces"
+
 do_install:append () {
     rm ${D}/etc/network/interfaces
+    install -d ${D}/var/etc/
+    install -d ${D}/var/etc/network
+    install -m 0644 ${WORKDIR}/interfaces ${D}/var/etc/network
 }