[tls] Clean up change cipher spec record handling

Define and use data structures and constants for the (single-byte)
change cipher spec records.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/tls.h

@@ -52,6 +52,9 @@ struct tls_header {
 /** Change cipher content type */
 #define TLS_TYPE_CHANGE_CIPHER 20
 
+/** Change cipher spec magic byte */
+#define TLS_CHANGE_CIPHER_SPEC 1
+
 /** Alert content type */
 #define TLS_TYPE_ALERT 21