MdeModulePkg: IP4 should re-initiate a DHCP if it detects network reconnection

v2:
* Update the MediaPresent detect declaring.

IP4 driver should re-initiate a DHCP if it detects that there is a network.
To fix this issue, we can implement the DHCP re-initiate policy while the media
change detected. The Ip4 driver should set a timer to signal the Ip4 to run the
DHCP configuration again(D.O.R.A). IP4 driver should free old IP address related
resource, then initiate a DHCP process to acquire new IP.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Zhang Lubo <lubo.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Lubo Zhang <lubo.zhang@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18232 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Universal/Network/Ip4Dxe/Ip4Impl.h

@@ -203,6 +203,13 @@ struct _IP4_SERVICE {
 
   EFI_EVENT                       Timer;
 
+  EFI_EVENT                       ReconfigEvent;
+
+  //
+  // Underlying media present status. 
+  //
+  BOOLEAN                         MediaPresent;
+
   //
   // IPv4 Configuration II Protocol instance
   //