tests/external_module_dir: test for `EXTERNAL_MODULE_DIRS`

It demonstrates:

 * Adding a module with source code
 * Setting a header include directory
 * Adding dependences, which are evaluated before other modules dependencies

If the application compiles, everything is ok.

doc/doxygen/src/creating-modules.md

@@ -74,6 +74,9 @@ The external module can optionally define the following files:
   API headers include paths to the `USEMODULE_INCLUDES` variable.
 * `Makefile.dep` file to set module dependencies
 
+An example can be found in
+[`tests/external_module_dirs`](https://github.com/RIOT-OS/RIOT/tree/master/tests/external_module_dirs)
+
 Pseudomodules                                                  {#pseudomodules}
 =============
 Pseudomodules are modules that do not have any code. Their main use cases are

tests/external_module_dirs/Makefile

+APPLICATION = external_module_dirs
+BOARD ?= native
+RIOTBASE ?= $(CURDIR)/../..
+
+USEMODULE += random
+
+USEMODULE += external_module
+EXTERNAL_MODULE_DIRS += $(CURDIR)/external_module
+
+include $(RIOTBASE)/Makefile.include

tests/external_module_dirs/README.md

+Test of `EXTERNAL_MODULE_DIRS` handling
+=======================================
+
+This is a test for the `EXTERNAL_MODULE_DIRS` variable.
+
+It demonstrates:
+
+ * Adding a module with source code
+ * Setting a header include directory
+ * Adding dependencies, which are evaluated before other modules dependencies
+
+If the application compiles, everything is ok.

tests/external_module_dirs/external_module/Makefile

+include $(RIOTBASE)/Makefile.base

tests/external_module_dirs/external_module/Makefile.dep

+USEMODULE += random
+# Set a different prng than the default prng_tinymt32
+USEMODULE += prng_xorshift

tests/external_module_dirs/external_module/Makefile.include

+# Use an immediate variable to evaluate `MAKEFILE_LIST` now
+USEMODULE_INCLUDES_external_module := $(abspath $(dir $(lastword $(MAKEFILE_LIST))))/include
+USEMODULE_INCLUDES += $(USEMODULE_INCLUDES_external_module)

tests/external_module_dirs/external_module/external_module.c

+/*
+ * Copyright (C) 2018 Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     tests
+ * @{
+ *
+ * @file
+ * @brief       Test the EXTERNAL_MODULE_DIRS feature
+ * @note        Define a shared variable
+ *
+ * @author      Gaëtan Harter <gaetan.harter@fu-berlin.de>
+ *
+ * @}
+ */
+
+#include "external_module.h"
+
+char *external_module_message = "Linking worked";

tests/external_module_dirs/external_module/include/external_module.h

+/*
+ * Copyright (C) 2018 Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @defgroup
+ * @ingroup
+ * @brief
+ * @{
+ *
+ * @file
+ * @brief
+ *
+ * @author  Gaëtan Harter <gaetan.harter@fu-berlin.de>
+ */
+#ifndef EXTERNAL_MODULE_H
+#define EXTERNAL_MODULE_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief   A simple string message
+ */
+extern char *external_module_message;
+
+#ifdef __cplusplus
+}
+#endif
+
+/** @} */
+#endif /* EXTERNAL_MODULE_H */

tests/external_module_dirs/main.c

+/*
+ * Copyright (C) 2018 Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     tests
+ * @{
+ *
+ * @file
+ * @brief       Test the EXTERNAL_MODULE_DIRS feature
+ *
+ * @author      Gaëtan Harter <gaetan.harter@fu-berlin.de>
+ *
+ * @}
+ */
+
+#include <stdio.h>
+
+#include "external_module.h"
+
+#ifdef MODULE_PRNG_TINYMT32
+#error "Error: it included the default dependency"
+#endif
+
+#ifndef MODULE_PRNG_XORSHIFT
+#error "Dependency not included"
+#endif
+
+int main(void)
+{
+    puts("If it compiles, it works!");
+    printf("Message: %s\n", external_module_message);
+    return 0;
+}