NetworkPkg/SnpDxe: Add PCD to remove ExitBootServices event from SNP driver.

NetworkPkg/SnpDxe: Add PCD to remove ExitBootServices event from SNP driver.

Patch addresses Bugzilla #1974.

During ExitBootServices stage, drivers should not call any
functions known to use Memory Allocation Services. One of such
functions (as per UEFI spec) is UNDI->Shutdown().

Since UNDI drivers during ExitBootServices phase are expected
to put the adapter to such a state that it will not perform any DMA
operations, there is no need to interface UNDI by SNP driver during
that phase.

Finally, since ExitBootServices event notification function in SNP
only calls UNDI->Shutdown() and Stop() functions, there is no need
to create this event at all. Adding PCD to control creation of event
reacting to ExitBootServices() call so that systems with UNDIs relying
on SNP to call their Shutdown() and Stop() can still work.

Change-Id: Idd76f26d2e8ff7cf88b2d75e2d524c74211f2e89
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>
Signed-off-by: Maciej Rabeda <maciej.rabeda@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>