BaseTools: Clean the ffs folder before generating files in it

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2965

The content in Guid.xref depends on the files under the corresponding
ffs folder.(refer to the commit 5e9256cd7f)
To make Guid.xref update in the incremental build,
clean the files under that ffs folder before generating files in it.

Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>

Reviewed-by: Yuwei Chen<yuwei.chen@intel.com>

BaseTools/Source/Python/GenFds/FfsFileStatement.py

@@ -21,6 +21,7 @@ from .GuidSection import GuidSection
 from .FvImageSection import FvImageSection
 from .Ffs import FdfFvFileTypeToFileType
 from .GenFdsGlobalVariable import GenFdsGlobalVariable
+import shutil
 
 ## generate FFS from FILE
 #
@@ -67,6 +68,8 @@ class FileStatement (FileStatementClassObject):
         if FvName:
             Str += FvName
         OutputDir = os.path.join(GenFdsGlobalVariable.FfsDir, Str)
+        if os.path.exists(OutputDir):
+            shutil.rmtree(OutputDir)
         if not os.path.exists(OutputDir):
             os.makedirs(OutputDir)