For network dynamic media support:

1. add library function NetLibDetectMedia to NetLib for media detection
2. update MnpDxe to periodically poll for media status update and check for media status before packet transmit
3. update Ip4Dxe to return ModeData using Mnp->GetModeData()
4. update IScsiDxe to check for media status before try to do DHCP and session login
5. update UefiPxeBcDxe to check for media status before PXE start

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@9919 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Universal/Network/Dhcp4Dxe/Dhcp4Impl.c

@@ -770,6 +770,7 @@ ON_EXIT:
   @retval EFI_ALREADY_STARTED   Some other EFI DHCPv4 Protocol instance already started the
                                 DHCP process.
   @retval EFI_DEVICE_ERROR      An unexpected system or network error occurred.
+  @retval EFI_NO_MEDIA          There was a media error.
 
 **/
 EFI_STATUS