BaseTools: Fixed a bug of IgnoreAutoGen

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2080
After checking that if the build can't
ignore Autogen due to there is no compelet autogen files,
the build tool need to do a completely Autogen.

This patch is to fix a bug that if AutoGen
can't be skiped, the SkipAutoGen flag
need to set to False

Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/Python/build/build.py

@@ -2204,6 +2204,7 @@ class Build():
                 if self.SkipAutoGen:
                     Wa = self.VerifyAutoGenFiles()
                     if Wa is None:
+                        self.SkipAutoGen = False
                         Wa, self.BuildModules = self.PerformAutoGen(BuildTarget,ToolChain)
                     else:
                         GlobalData.gAutoGenPhase = True