NetworkPkg: Add PCD to enable the HTTP connections switch

v3:
* Correct the commits grammar

v2:
* Rename the PCD to PcdAllowHttpConnections.
* Refine the PCD descriptions.

If the value of PcdAllowHttpConnections is TRUE, HTTP connections are
allowed. Both the "https://" and "http://" URI schemes are permitted.
Otherwise, HTTP connections are denied. Only the "https://" URI scheme
is permitted.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Kinney Michael D <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Gary Lin <glin@suse.com>
Tested-by: Gary Lin <glin@suse.com>

NetworkPkg/HttpDxe/HttpImpl.c

@@ -1,7 +1,7 @@
 /** @file
   Implementation of EFI_HTTP_PROTOCOL protocol interfaces.
 
-  Copyright (c) 2015 - 2016, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2015 - 2017, Intel Corporation. All rights reserved.<BR>
   (C) Copyright 2015-2016 Hewlett Packard Enterprise Development LP<BR>
 
   This program and the accompanying materials
@@ -354,6 +354,16 @@ EfiHttpRequest (
     //
     HttpInstance->UseHttps = IsHttpsUrl (Url);
 
+    //
+    // HTTP is disabled, return directly if the URI is not HTTPS.
+    //
+    if (!PcdGetBool (PcdAllowHttpConnections) && !(HttpInstance->UseHttps)) {
+      
+      DEBUG ((EFI_D_ERROR, "EfiHttpRequest: HTTP is disabled.\n"));
+
+      return EFI_ACCESS_DENIED;
+    }
+
     //
     // Check whether we need to create Tls child and open the TLS protocol.
     //