Allow chip-tool echo-ble to work along with credential passing (#1965)

Prior to #1898, received bluetooth messages were echoed back
to the sender, and this facility is used by the command-line
`chip-tool echo-ble`. To restore that, this change adopts a
stricter format for WiFi credentials and treats anything else
as an echo request. (This too is temporary code, pending an
actual rendezvous implementation.)

examples/wifi-echo/server/esp32/main/RendezvousSession.cpp

@@ -81,8 +81,25 @@ void RendezvousSession::HandleMessageReceived(Ble::BLEEndPoint * endPoint, Packe
 
     ChipLogProgress(Ble, "RendezvousSession: Receive message: %s", msg);
 
-    char * ssid = strtok(msg, ":");
-    char * key  = strtok(NULL, ":");
-    ChipLogProgress(Ble, "RendezvousSession: SSID: %s, key: %s", ssid, key);
-    SetWiFiStationProvisioning(ssid, key);
+    if ((bufferLen > 3) && (msg[0] == msg[1]) && (msg[0] == msg[bufferLen - 1]))
+    {
+        // WiFi credentials, of the form ‘::SSID:password:’, where ‘:’ can be any single ASCII character.
+        msg[1]      = 0;
+        char * ssid = strtok(&msg[2], msg);
+        char * key  = strtok(NULL, msg);
+        if (ssid && key)
+        {
+            ChipLogProgress(Ble, "RendezvousSession: SSID: %s, key: %s", ssid, key);
+            SetWiFiStationProvisioning(ssid, key);
+        }
+        else
+        {
+            ChipLogError(Ble, "RendezvousSession: SSID: %p, key: %p", ssid, key);
+        }
+    }
+    else
+    {
+        // Echo.
+        mEndPoint->Send(buffer);
+    }
 }

src/darwin/CHIPTool/CHIPTool/QRCode/QRCodeViewController.m

@@ -260,7 +260,7 @@ - (void)retrieveAndSendWifiCredentials
 
 - (void)sendWifiCredentialsWithSSID:(NSString *)ssid password:(NSString *)password
 {
-    NSString * msg = [NSString stringWithFormat:@"%@:%@", ssid, password];
+    NSString * msg = [NSString stringWithFormat:@"::%@:%@:", ssid, password];
     NSError * error;
     BOOL didSend = [self.chipController sendMessage:[msg dataUsingEncoding:NSUTF8StringEncoding] error:&error];
     if (!didSend) {