Clarify the U2F Attestation format to have a single certificate

The [U2F Raw Message Format](https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-u2f-raw-message-formats-v1.2-ps-20170411.html#h3_registration-response-message-success) only allows for a single attestation certificate in U2F responses.

This PR reflects this in the U2F Attestation Format to reduce the chance of misunderstanding when implementing the server verification.

index.bs

@@ -3494,16 +3494,15 @@ This attestation statement format is used with FIDO U2F authenticators using the
                       )
 
     u2fStmtFormat = {
-                        x5c: [ attestnCert: bytes, * (caCert: bytes) ],
+                        x5c: [ attestnCert: bytes ],
                         sig: bytes
                     }
     ```
 
     The semantics of the above fields are as follows:
 
     : x5c
-    :: The elements of this array contain the attestation certificate and its certificate chain, each encoded in X.509 format.
-        The attestation certificate MUST be the first element in the array.
+    :: A single element array containing the attestation certificate in X.509 format.
 
     : sig
     :: The [=attestation signature=].  
@@ -3527,7 +3526,7 @@ This attestation statement format is used with FIDO U2F authenticators using the
     as follows:
     1. Verify that |attStmt| is valid CBOR conforming to the syntax defined above and perform CBOR decoding on it to extract the
         contained fields.
-    1. Let |attCert| be the value of the first element of |x5c|. Let |certificate public key| be the public key
+    1. Check that |x5c| has exactly one element and let |attCert| be that element. Let |certificate public key| be the public key
         conveyed by |attCert|. If |certificate public key| is not an Elliptic Curve (EC) public 
         key over the P-256 curve, terminate this algorithm and return an appropriate error. 
     1. Extract the claimed |rpIdHash| from |authenticatorData|, and the claimed |credentialId| and |credentialPublicKey| from