Permit specification of extra bootstrap packages

This change permits the yum-minimal element to be used in downstream
custom distributions, which may have additional packages containing repo
config or GPG keys needed.

This could also be utilized at a later time to move the
distribution-specific logic in this method to each distribution element
separately.

Change-Id: Ic1434bb2fe7301086cf11ba6bd7f2ee187c5e6c8

diskimage_builder/elements/yum-minimal/README.rst

@@ -22,3 +22,8 @@ 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.
+
+If you are bootstrapping a custom or unsupported OS, and need to install
+additional packages to setup the base chroot, set
+``DIB_YUM_MINIMAL_BOOTSTRAP_PACKAGES`` to the list of additional packages to
+install.

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

@@ -71,8 +71,8 @@ function _install_repos {
     # is that they are only installed in one big transaction with the
     # rest of the system?  but we don't want to use yum to do this
     # (see above) so ...
-    packages="basesystem filesystem setup "
-
+    packages="${DIB_YUM_MINIMAL_BOOTSTRAP_PACKAGES:-} "
+    packages+="basesystem filesystem setup "
     if [[ ${DISTRO_NAME} = fedora && ${DIB_RELEASE} -gt 29 ]]; then
         packages+="fedora-release-cloud fedora-release-common "
     else

releasenotes/notes/specify-bootstrap-packages-via-env-22f5c6982c09f16a.yaml

@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    Now, users of the ``yum-minimal`` element can specify additional packages
+    to install while creating the initial chroot by setting
+    ``DIB_YUM_MINIMAL_BOOTSTRAP_PACKAGES``. This can be useful for adding
+    support for new linux distributions that use yum.
+