NetworkPkg: Enable MD5 while enable iSCSI

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3003

There is a plan to make MD5 disable as default.
The new MACRO ENABLE_MD5_DEPRECATED_INTERFACES
would be introduced to enable MD5. Make the
definition ahead of the change to avoid build
error after the MACRO changed.
1. Add the NetworkBuildOptions.dsc.inc to define
the MACRO for build (support: GCC, INTEL, MSFT and
RVCT)
2. Add the BuildOption file to Network.dsc.inc

Cc: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
Message-Id: <20201112055558.2348-5-zhichao.gao@intel.com>
[lersek@redhat.com: clean up comments in "NetworkBuildOptions.dsc.inc"]
[lersek@redhat.com: hoist "BuildOptions" above "Components" in
 "Network.dsc.inc" for bug compat with edk2-platforms]
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>

NetworkPkg/NetworkBuildOptions.dsc.inc

@@ -0,0 +1,22 @@
+## @file
+# Network DSC include file for [BuildOptions] sections of all Architectures.
+#
+# This file can be included in the [BuildOptions*] section(s) of a platform DSC file
+# by using "!include NetworkPkg/NetworkBuildOptions.dsc.inc", to specify the C language
+# feature test macros (eg., API deprecation macros) according to the flags described
+# in "NetworkDefines.dsc.inc".
+#
+# Supported tool chain families: "GCC", "INTEL", "MSFT", "RVCT".
+#
+# Copyright (c) 2020, Intel Corporation. All rights reserved.<BR>
+#
+#    SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+##
+
+!if $(NETWORK_ISCSI_ENABLE) == TRUE
+  MSFT:*_*_*_CC_FLAGS = /D ENABLE_MD5_DEPRECATED_INTERFACES
+  INTEL:*_*_*_CC_FLAGS = /D ENABLE_MD5_DEPRECATED_INTERFACES
+  GCC:*_*_*_CC_FLAGS = -D ENABLE_MD5_DEPRECATED_INTERFACES
+  RVCT:*_*_*_CC_FLAGS = -DENABLE_MD5_DEPRECATED_INTERFACES
+!endif