MdeModulePkg: Fix issue about current Ip4Dxe implementation for DHCP DORA process

MdeModulePkg: Fix issue about current Ip4Dxe implementation for DHCP DORA process

DHCP policy is applied as default at boot time on all NICs in the system, which results
in all NIC ports attempting DHCP and trying to acquire IP addresses during boot.
Ip4 driver should only set dhcp as default policy, and not trigger DORA at driver binding
start(). We should start DORA until one IP child is configured to use default address.

Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Ye Ting <ting.ye@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>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18201 6f19259b-4bc3-4df7-8a09-765794883524