Allow extra repositories to be added to images

This patch adds option DIB_YUM_MINIMAL_EXTRA_REPOS to yum-minimal to
allow DIB users to include extra repositories to their final image.

Change-Id: I89549f4b0f4c9470143b5064817acab5043e31c5

diskimage_builder/elements/yum-minimal/README.rst

@@ -18,3 +18,7 @@ If you wish to build from specific mirrors, set
 files to use during bootstrap and build.  The repo files should be
 named with a prefix ``dib-mirror-`` and will be removed from the final
 image.
+
+If you wish to include extra repositories, set ``DIB_YUM_MINIMAL_EXTRA_REPOS``
+to a directory with the ``.repo`` files. The repo files will not be removed
+from the final image.

diskimage_builder/elements/yum-minimal/root.d/08-yum-chroot

@@ -128,6 +128,10 @@ function _install_repos {
             $TARGET_ROOT/etc/yum.repos.d/
     fi
 
+    if [[ -n ${DIB_YUM_MINIMAL_EXTRA_REPOS:-} ]]; then
+        sudo cp ${DIB_YUM_MINIMAL_EXTRA_REPOS}/* \
+            $TARGET_ROOT/etc/yum.repos.d/
+    fi
 }
 
 # _install_pkg_manager packages...