IntelFsp2Pkg: FSP can utilize bootloader stack

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1485

Current FSP utilizes pre-allocated temporary memory from
boot loader for both heap and stack. To reduce overall
temporary memory usage FSP may share the same stack with
boot loader and only needs a smaller memory for heap,
no separate memory required for stack.
Setting PcdFspHeapSizePercentage to 0 to enable FSP sharing
stack with boot loader, in this case boot loader stack
has to be large enough for FSP to use. Default is 50
(half memory heap and half memory stack) for backward
compatible with original model.

Test: Verified on internal platform and booting successfully
      with both modes.

Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>

IntelFsp2Pkg/FspSecCore/SecMain.h

@@ -135,4 +135,17 @@ ProcessLibraryConstructorList (
   VOID
   );
 
+/**
+
+  Return value of esp.
+
+  @return  value of esp.
+
+**/
+UINT32
+EFIAPI
+AsmReadEsp (
+  VOID
+  );
+
 #endif