BaseTools: align ERROR/WARNING/RETURN macros with MdePkg versions

BaseTools' BaseTypes.h defined the ENCODE_ERROR macro as
 #define ENCODE_ERROR(a)              ((RETURN_STATUS)(MAX_BIT | (a)))
whereas MdePkg defines it as
 #define ENCODE_ERROR(StatusCode)     ((RETURN_STATUS)(MAX_BIT | (StatusCode)))

When building with GCC 6.3 (at least) the former triggers
"error: overflow in implicit constant conversion [-Werror=overflow]"
Resolve this by aligning it with the latter one.

This also requires aligning the BaseTools typedef of RETURN_STATUS with
the MdePkg one: INTN -> UINTN.

While at it, update adjacent ENCODE_WARNING and RETURN_ERROR as well.

Add an explicit initialization of *Alignment to 0 in GenFfs.c
GetAlignmentFromFile to get rid of a warning occuring with GCC after
this change (-Werror=maybe-uninitialized).

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/C/GenFfs/GenFfs.c

@@ -529,6 +529,7 @@ GetAlignmentFromFile(char *InFile, UINT32 *Alignment)
 
   InFileHandle        = NULL;
   PeFileBuffer        = NULL;
+  *Alignment          = 0;
 
   memset (&ImageContext, 0, sizeof (ImageContext));