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 5e9256cd)
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&lt;yuwei.chen@intel.com>