Allow injecting arbitrary yum repo configuration

Allows specifying the path to a yum repo configuration file to be used
during the image build process. The repo configuration file is removed
at the end of the build. This is useful in that it allows using any
arbitrary repo configuration during an image build.

Change-Id: I7d5c67d74a0bb4722ffc60aacfd9fa7e80fb59d5

elements/yum/README.md

@@ -8,3 +8,8 @@ chroot so that they can be reused by subsequent image builds.  The cache
 increases image building speed when building multiple images, especially on
 slow connections.  This is more effective than using an HTTP proxy as a yum
 cache since the same rpm from different mirrors is often requested.
+
+A custom yum repository configuration can also be applied by defining
+`DIB_YUM_REPO_CONF` to the path to a repo configuration file. The file will
+be copied to /etc/yum.repos.d/dib-yum-repo-conf.repo during the image build,
+and then removed at the end of the build.

elements/yum/cleanup.d/99-remove-yum-repo-conf

@@ -0,0 +1,6 @@
+#!/bin/bash
+
+set -eux
+set -o pipefail
+
+sudo rm -f $TMP_MOUNT_PATH/etc/yum.repos.d/dib-yum-repo-conf.repo

elements/yum/extra-data.d/99-yum-repo-conf

@@ -0,0 +1,18 @@
+#!/bin/bash
+# Add an additional yum repo configuration with $DIB_YUM_REPO_CONF
+
+set -eux
+set -o pipefail
+
+# exit directly if DIB_YUM_REPO_CONF is not defined properly
+if [ -z "${DIB_YUM_REPO_CONF:-}" ] ; then
+    echo "DIB_YUM_REPO_CONF is not set - no repo configuration will be copied in"
+    exit 0
+elif [ ! -f "$DIB_YUM_REPO_CONF" ] ; then
+    echo "DIB_YUM_REPO_CONF is not a valid yum repo configuration file."
+    echo "You should assign a proper yum repo configuration file in DIB_YUM_REPO_CONF"
+    exit 1
+fi
+
+# copy the yum repo configuration
+sudo cp -L -f $DIB_YUM_REPO_CONF $TMP_MOUNT_PATH/etc/yum.repos.d/dib-yum-repo-conf.repo