ARM: boot: Add an implementation of strnlen for libfdt

Recent versions of libfdt add a dependency on strnlen. Copy the
implementation in lib/string.c here, so we can update libfdt.
Acked-by: NRussell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: NRob Herring <robh@kernel.org>

arch/arm/boot/compressed/string.c

@@ -65,6 +65,15 @@ size_t strlen(const char *s)
 	return sc - s;
 }
 
+size_t strnlen(const char *s, size_t count)
+{
+	const char *sc;
+
+	for (sc = s; count-- && *sc != '\0'; ++sc)
+		/* nothing */;
+	return sc - s;
+}
+
 int memcmp(const void *cs, const void *ct, size_t count)
 {
 	const unsigned char *su1 = cs, *su2 = ct, *end = su1 + count;