NetworkPkg: Add DSC/FDF include segment files to NetworkPkg.

NetworkPkg: Add DSC/FDF include segment files to NetworkPkg.

This patch provides a set of include segment files for platform owner to
easily enable/disable network stack support on their platform.

For DSC, there are:
- a "NetworkDefines.dsc.inc" for the [Defines] section(s),
- a "NetworkLibs.dsc.inc" for the [LibraryClasses*] section(s),
- a "NetworkPcds.dsc.inc" for the [Pcds*] section(s),
- a "NetworkComponents.dsc.inc" for the [Components*] section(s).
For FDF, there is:
- a "Network.fdf.inc" for the [Fv*] section(s).

These files can be added to the platform DSC/FDF file by using
  !include NetworkPkg/xxx
where "xxx" is the *.inc file name.

A platform DSC file can diverge from the defaults in
"NetworkDefines.dsc.inc" by setting the individual DEFINEs before
including "NetworkDefines.dsc.inc".
And, build command line ("-D FLAG=VALUE") can be used to enable or
disable related feature set, please check "NetworkDefines.dsc.inc"
for a detail description of each flag.

The default value of these flags are:
  DEFINE NETWORK_ENABLE                 = TRUE
  DEFINE NETWORK_SNP_ENABLE             = TRUE
  DEFINE NETWORK_IP4_ENABLE             = TRUE
  DEFINE NETWORK_IP6_ENABLE             = TRUE
  DEFINE NETWORK_TLS_ENABLE             = TRUE
  DEFINE NETWORK_HTTP_BOOT_ENABLE       = TRUE
  DEFINE NETWORK_ALLOW_HTTP_CONNECTIONS = FALSE
  DEFINE NETWORK_ISCSI_ENABLE           = TRUE
  DEFINE NETWORK_VLAN_ENABLE            = TRUE

Related BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1293

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Ting Ye <ting.ye@intel.com>
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>