tests: add a test for buildir integration with docker

Add a test to verify that the build system correctly integrates 'build'
with docker even if set outside of RIOTBASE.

tests/build_system_builddir_docker/Makefile

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+# Only one board the goal is not the compilation but the build system
+# integration
+BOARD_WHITELIST = native
+
+include ../Makefile.tests_common
+include $(RIOTBASE)/Makefile.include
+
+
+.PHONY: test_file_in_build clean_file_in_build
+
+
+TESTFILE = $(BUILD_DIR)/tmp/output_$(APPLICATION)
+
+# Create the file as a dependency of an intermediate target
+# This ensures it is executed in docker
+$(ELFFILE): | $(TESTFILE)
+test: test_file_in_build
+clean: clean_file_in_build
+
+
+$(TESTFILE): Makefile  # Rebuild if this Makefile changed
+	$(Q)mkdir -p $(dir $(@))
+	$(Q)echo '$(APPLICATION)/$(BOARD)' > $@
+
+test_file_in_build:
+	test -f $(TESTFILE) && test $$(cat $(TESTFILE)) = '$(APPLICATION)/$(BOARD)'
+
+clean_file_in_build:
+	$(Q)rm -rf $(dir $(TESTFILE))

tests/build_system_builddir_docker/README.md

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+Build system test: build directory
+==================================
+
+Test handling of the build directory within docker.
+
+This test is mainly done to test 'docker' integration to verify that the
+`BUILD_DIR` variable is correctly passed and mounted into docker.
+
+
+Usage
+-----
+
+The test can be run with
+
+    make clean all test \
+        BUILD_DIR=/tmp/riot/build \
+        BUILD_IN_DOCKER=1 DOCKER='sudo docker'
+
+I used a directory outside of 'RIOT' to show that the directory is mounted in
+the container.
+A directory with an absolute path in RIOT should also fail but we can use a
+different one anyway.
+
+It can also be run without docker but does not really test anything.

tests/build_system_builddir_docker/main.c

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+/*
+ * 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       Empty application
+ *
+ * @author      Gaëtan Harter <[email protected]>
+ *
+ * @}
+ */
+
+int main(void)
+{
+    /*
+     * The main part of the test is in the Makefile. See README.md for more
+     * informations
+     */
+    return 0;
+}