Add yum element.

The yum element provides for specific customizations
for yum based distros.  The included customization is
a yum cache mounted outside of the chroot so that yum
downloaded packages are reused on subsequent image builds.

Change-Id: I6833c9fdbc83cb09debec6a789082e105c917800

elements/fedora/element-deps

@@ -2,3 +2,4 @@ dib-run-parts
 dracut-network
 cache-url
 dkms
+yum

elements/yum/README.md

@@ -0,0 +1,10 @@
+Provide yum specific image building glue.
+
+RHEL/Fedora/CentOS and other yum based distributions need specific yum
+customizations.
+
+Customizations include caching of downloaded yum packages outside of the build
+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.

elements/yum/pre-install.d/01-yum-keepcache

@@ -0,0 +1,8 @@
+#!/bin/bash
+
+set -e
+
+[ -n "TARGET_ROOT" ]
+
+sudo sed -i 's/keepcache=0/keepcache=1/' $TARGET_ROOT/etc/yum.conf
+sudo sed -i 's/cachedir=\/var\/cache\/yum/cachedir=\/tmp\/yum/' $TARGET_ROOT/etc/yum.conf

elements/yum/root.d/50-yum-cache

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+set -e
+
+[ -n "TARGET_ROOT" ]
+
+YUM_CACHE_DIR=~/.cache/image-create/yum
+mkdir -p $YUM_CACHE_DIR
+
+sudo mkdir -p $TMP_MOUNT_PATH/tmp/yum
+sudo mount --bind $YUM_CACHE_DIR $TMP_MOUNT_PATH/tmp/yum