libc: Import from TF-A

Based on arm-trusted-firware commit 873e394b3bf93214a441f9f98237b58fbbea55aa

Change-Id: I510e092f2b9ff333e9461bdde8d80ed1fab1460c
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
diff --git a/lib/libc/exit.c b/lib/libc/exit.c
new file mode 100644
index 0000000..f4ffe27
--- /dev/null
+++ b/lib/libc/exit.c
@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2015-2018, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <stdlib.h>
+
+static void (*exitfun)(void);
+
+void exit(int status)
+{
+	if (exitfun != NULL)
+		(*exitfun)();
+	for (;;)
+		;
+}
+
+int atexit(void (*fun)(void))
+{
+	if (exitfun != NULL)
+		return -1;
+	exitfun = fun;
+
+	return 0;
+}