MdeModulePkg/Variable/RuntimeDxe: introduce MorLockInitAtEndOfDxe() hook

MdeModulePkg/Variable/RuntimeDxe: introduce MorLockInitAtEndOfDxe() hook

Introduce the MorLockInitAtEndOfDxe() hook, in order to allow
MorLockInit() to delay / queue operations until EndOfDxe. (Or, if the
platform never signals EndOfDxe, until ReadyToBoot.)

Call MorLockInitAtEndOfDxe() whenever we set "mEndOfDxe" to TRUE:

- in VariableRuntimeDxe:
  - in the OnReadyToBoot() function,
  - in the OnEndOfDxe() function;

- in VariableSmm:
  - on the SMM_VARIABLE_FUNCTION_READY_TO_BOOT SMI request,
  - in the SmmEndOfDxeCallback() function.

For now, implement MorLockInitAtEndOfDxe() as a no-op in both
VariableRuntimeDxe and VariableSmm.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Ladi Prosek <lprosek@redhat.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Tested-by: Ladi Prosek <lprosek@redhat.com>