Add select-boot-kernel-initrd element

Encapsulate the new select-boot-kernel-initrd common function
into an element so other elements can make use of it.

Change-Id: Ie587bd5dae1afebfd5646190dfa951a7949938cd

elements/select-boot-kernel-initrd/README.md

@@ -0,0 +1,5 @@
+A helper script to get the kernel and initrd image.
+
+It uses the function select_boot_kernel_initrd from the library img-functions
+to find the newest kernel and ramdisk in the image, and returns them as a
+concatenated string separating the values with a colon (:).

elements/select-boot-kernel-initrd/bin/select-boot-kernel-initrd

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+#!/bin/bash
+
+# Copyright 2012 Hewlett-Packard Development Company, L.P.
+# Copyright 2013 Red Hat, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+set -eu
+set -o pipefail
+
+source /lib/diskimage-builder/img-functions
+
+select_boot_kernel_initrd /
+
+echo ${KERNEL}:${RAMDISK}

elements/select-boot-kernel-initrd/cleanup.d/99-remove-dib-img-functions

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+#!/bin/bash
+
+set -eu
+set -o pipefail
+
+sudo rm -f $TMP_MOUNT_PATH/lib/diskimage-builder/img-functions
+sudo rmdir $TMP_MOUNT_PATH/lib/diskimage-builder

elements/select-boot-kernel-initrd/extra-data.d/99-copy-dib-img-functions

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+#!/bin/bash
+# Copy elements-functions into chroot so we can access the get_kernel_image function
+
+set -eu
+set -o pipefail
+
+sudo mkdir -p $TMP_MOUNT_PATH/lib/diskimage-builder
+sudo cp -t $TMP_MOUNT_PATH/lib/diskimage-builder $_LIB/img-functions