MdeModulePkg: Add PcdEmuVariableNvModeEnable in dec

MdeModulePkg: Add PcdEmuVariableNvModeEnable in dec

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1323
Merge EmuVariable and Real variable driver.

Add PcdEmuVariableNvModeEnable (support both static and
dynamic) to indicate if Variable driver will enable
emulated variable NV mode.

This patch prepares for adding emulated variable NV mode
support in VariableRuntimeDxe.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>