CorebootPayloadPkg: Use merged variable driver for emulated NV mode

CorebootPayloadPkg: Use merged variable driver for emulated NV mode

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

The real variable driver has been updated to support emulated
variable NV mode and the EmuVariableRuntimeDxe will be removed
later, so use merged variable driver for emulated NV mode.

Cc: Maurice Ma <maurice.ma@intel.com>
Cc: Prince Agyeman <prince.agyeman@intel.com>
Cc: Benjamin You <benjamin.you@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Benjamin You <benjamin.you@intel.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>