Merge pull request #283

83ec9d3 update unit tests (douglasbakkum)
269b532 async command processing for hijack mode (douglasbakkum)
3c548fd refactor naming 'send' -> 'queue' where more accurate (douglasbakkum)
eefda0f u2f.c: adopt v2 `_` notation for static var/func (douglasbakkum)
2edc3ec u2f hijack bugfix: empty return frame when need more data (douglasbakkum)

src/u2f/u2f.h

@@ -20,6 +20,7 @@
 #define U2F_MAX_ATT_CERT_SIZE   1024    // Max size of attestation certificate
 #define U2F_MAX_EC_SIG_SIZE     72      // Max size of ANS.1 DER encoded EC signature
 #define U2F_CTR_SIZE            4       // Size of counter field
+#define U2F_FRAME_SIZE (3 + U2F_CTR_SIZE)// 1-byte flag | 4-byte counter | 2-byte status
 #define U2F_APPID_SIZE          32      // Size of application id
 #define U2F_NONCE_LENGTH        32      // Size of challenge nonce
 #define U2F_UNCOMPRESSED_POINT  0x04    // Uncompressed point format