libc: Introduce cdefs.h, assert.h and strlen.c

Change-Id: I76091d52571f1950111c4b1670d5fc3883607715
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

lib/libc/strlen.c

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2018, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <string.h>
+
+size_t strlen(const char *s)
+{
+	const char *cursor = s;
+
+	while (*cursor)
+		cursor++;
+
+	return cursor - s;
+}