stdio_semihosting: Add shell-based test application

tests/sys_stdio_semihosting/Makefile

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+include ../Makefile.tests_common
+
+USEMODULE += stdio_semihosting
+USEMODULE += shell
+USEMODULE += shell_commands
+USEMODULE += ps
+
+RIOT_TERMINAL = semihosting
+
+include $(RIOTBASE)/Makefile.include

tests/sys_stdio_semihosting/README.md

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+# Semihosting STDIO test
+
+This test aims to test ARM semihosting based STDIO on RIOT. The test is manual,
+starting a debugger on the CI for automated test is not supported.
+
+## Usage
+
+1. Flash the test on a board with an ARM-based MCU.
+2. Use `make term RIOT_TERMINAL=semihosting` to start a debug session with
+   semihosting enabled.
+3. Restart the microcontroller via the debug session.
+
+The shell prompt should be available in the GDB session.

tests/sys_stdio_semihosting/main.c

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+/*
+ * Copyright (C) 2020 Koen Zandberg
+ *
+ * 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       STDIO Semihosting test application using simple shell
+ *              interaction
+ *
+ * @author      Koen Zandberg <koen@bergzand.net>
+ *
+ * @}
+ */
+
+#include <stdio.h>
+
+#include "shell.h"
+#include "shell_commands.h"
+
+int main(void)
+{
+    puts("STDIO semihosting test application");
+
+    char line_buf[SHELL_DEFAULT_BUFSIZE];
+    shell_run(NULL, line_buf, SHELL_DEFAULT_BUFSIZE);
+
+    return 0;
+}