Add EditorConfig configuration

EditorConfig (https://editorconfig.org/) defines a file format for
defining coding styles. The most common editor do either provide native
support or via plugin support for EditorConfig.

This patch adds an EditorConfig configuration for s390-tools.

Acked-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

.editorconfig

@@ -0,0 +1,13 @@
+# Check https://editorconfig.org for details
+
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+indent_style = tab
+tab_width = 8
+
+[*.sh]
+shell_variant = bash  # used by `shfmt`