dib-lint: test elements have README.rst file

Add a basic test to ensure that all elements have a README.rst file.
This way they will be exhaustively listed in the Sphinx documentation.

Add dummy README.rst for 'disable-selinux' and 'rpm-distro' elements.

Change-Id: Ia5252ddd89b5ae5c6e9a12a66ef10f912fd54da5

bin/dib-lint

@@ -71,6 +71,11 @@ rc=0
 TMPDIR=$(mktemp -d /tmp/tmp.XXXXXXXXXX)
 trap "rm -rf $TMPDIR" EXIT
 
+# Elements have a README.rst file
+for i in $(find $ELEMENTS_DIR -mindepth 1 -maxdepth 1 -type d); do
+    [ -f "${i}/README.rst" ] || error "$i lacks a README.rst"
+done
+
 # note .py files are run through flake8 directly in tox.ini
 for i in $(find $ELEMENTS_DIR -type f \
                 -not -name \*.rst     \

diskimage_builder/elements/disable-selinux/README.rst

@@ -0,0 +1,4 @@
+disable-selinux
+---------------
+
+Including this element disables SELinux.

diskimage_builder/elements/rpm-distro/README.rst

@@ -0,0 +1,4 @@
+rpm-distro
+----------
+
+Base element for distributions based on RPM.