PBL: Add strnlen, needed for printf support

vsprintf needs strnlen, so in oder to add console support to
the PBL we need a strnlen implementation.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

pbl/string.c

@@ -119,3 +119,17 @@ void *memset(void *s, int c, size_t count)
 		*xs++ = c;
 	return s;
 }
+
+/**
+ * strnlen - Find the length of a length-limited string
+ * @s: The string to be sized
+ * @count: The maximum number of bytes to search
+ */
+size_t strnlen(const char * s, size_t count)
+{
+	const char *sc;
+
+	for (sc = s; count-- && *sc != '\0'; ++sc)
+		/* nothing */;
+	return sc - s;
+}