MdeModulePkg Variable: Add emulated variable NV mode support

MdeModulePkg Variable: Add emulated variable NV mode support

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

Add emulated variable NV mode support in real variable driver.
Platform can configure PcdEmuVariableNvModeEnable statically
(build time) or dynamically (boot time) to support emulated
variable NV mode.

If PcdEmuVariableNvModeEnable is configured to dynamic, its
value should be set before Variable driver starts to work,
otherwise default value will take effect.

Then EmuVariableRuntimeDxe could be removed, the removal of
EmuVariableRuntimeDxe will be done after platforms are migrated
to use the merged variable driver.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>