Copy resolv.conf handling from diskimage-builder

The copy in finalise_base runs too late for us, leaving a stale
resolv.conf around (usually copied from the build environment).

Change-Id: I4b89119b8e2eded72973fa12879b1419542ba984

dib/ironic-ramdisk-base/cleanup.d/98-prepare-resolve-conf

@@ -0,0 +1,42 @@
+#!/bin/bash
+
+# dib-lint: disable=safe_sudo
+
+if [ "${DIB_DEBUG_TRACE:-0}" -gt 0 ]; then
+    set -x
+fi
+set -eu
+set -o pipefail
+
+# NOTE(dtantsur): this a copy of code from img-functions. We need it because it
+# runs too late when the image is already built.
+
+# Finalise resolv.conf
+#
+# NOTE(ianw): the /etc/resolv.conf.ORIG file is an
+# external interface; elements might put a resolv.conf they
+# want in the final image into this file.
+#
+# In create_base() we replaced/created the initial resolv.conf
+# inside the image with a copy of the "outside" version so that
+# resolving during the build will work.
+#
+# If that file has been replace with a symlink (resolvconf package
+# can do this), or marked immutable, then don't restore the
+# original version, just leave it alone.
+if [ -L $TMP_MOUNT_PATH/etc/resolv.conf ] || \
+       lsattr $TMP_MOUNT_PATH/etc/resolv.conf | grep '^....i' >/dev/null ; then
+    # We're keeping the contents of resolv.conf set in the elements,
+    # so remove the old saved file
+    sudo rm -f $TMP_MOUNT_PATH/etc/resolv.conf.ORIG
+else
+    # Remove the resolv.conf we created and put the original (or
+    # perhaps modified) version back.
+    sudo rm -f $TMP_MOUNT_PATH/etc/resolv.conf
+    # Note that we use -L and -f to test here as test (and bash [[)
+    # return false with -e if the link target does not exist.
+    if [ -L $TMP_MOUNT_PATH/etc/resolv.conf.ORIG ] || [ -f $TMP_MOUNT_PATH/etc/resolv.conf.ORIG ] ; then
+        sudo mv $TMP_MOUNT_PATH/etc/resolv.conf.ORIG $TMP_MOUNT_PATH/etc/resolv.conf
+    fi
+fi
+