em4x70: Add write key convenience function. Use real values in writekey/auth help text so people with blank tags can program a test key and test authentication.

include/em4x70.h

@@ -31,6 +31,9 @@ typedef struct {
     uint8_t rnd[7];
     uint8_t frnd[4];
 
+    // Used to write new key
+    uint8_t crypt_key[12];
+
 } em4x70_data_t;
 
 #endif /* EM4X70_H__ */