BaseTool: Fixed incremental rebuild issue.

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1540

This issue in introduced by commit
d943b0c339

To convert bytes to string, we need to use bytes.decode()
instead of using str(bytes).

If the source file is not a txt file, ignore that file.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/Python/AutoGen/GenMake.py

@@ -1045,14 +1045,14 @@ cleanlib:
                     EdkLogger.error("build", FILE_OPEN_FAILURE, ExtraData=F.Path + "\n\t" + str(X))
                 if len(FileContent) == 0:
                     continue
-
-                if FileContent[0] == 0xff or FileContent[0] == 0xfe:
-                    FileContent = FileContent.decode('utf-16')
-                else:
-                    try:
-                        FileContent = str(FileContent)
-                    except:
-                        pass
+                try:
+                    if FileContent[0] == 0xff or FileContent[0] == 0xfe:
+                        FileContent = FileContent.decode('utf-16')
+                    else:
+                        FileContent = FileContent.decode()
+                except:
+                    # The file is not txt file. for example .mcb file
+                    continue
                 IncludedFileList = gIncludePattern.findall(FileContent)
 
                 for Inc in IncludedFileList: