BaseTools: Fixed issue in MultiThread Genfds function

BaseTools: Fixed issue in MultiThread Genfds function

https://bugzilla.tianocore.org/show_bug.cgi?id=1450
In the Multiple thread Genfds feature, build tool generates
GenSec, GenFFS command in Makefile.

The Non-Hii Driver does not generate .offset file for uni string offset,
but the build tool has not knowledge about this in autogen phase. So
in this patch, I add a check in Makefile for GenSec command. If the GenSec
input file does not exist, the GenSec will not be called. And if GenSec
command is not called, its output file, which is also the input file of
GenFfs command, will also not exist.So for GenFfs command,
I add a new command parameter -oi which means
the input file is an optional input file which would not exist. so
that I can generate GenFfs command with "-oi" parameter in Makefile.

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>