BaseTools:Enhance the way to handling included dsc file

BaseTools:Enhance the way to handling included dsc file

BZ:https://bugzilla.tianocore.org/show_bug.cgi?id=2400

In Dsc Parser, included dsc file is parsed always no matter
if its condition is False

  gUefiOvmfPkgTokenSpaceGuid.test1|FALSE
!if gUefiOvmfPkgTokenSpaceGuid.test1 == FALSE
  !include OvmfPkg/test1.dsc
!else
  !include OvmfPkg/test2.dsc
!endif

The patch avoids processing redundant dsc files and improves
the way Tool handles them.

In the above case, since the conditional result is FALSE,
"test2.dsc" is not parsed.

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