BaseTools: Reset FdsGlobalVariable

BaseTools: Reset FdsGlobalVariable

https://bugzilla.tianocore.org/show_bug.cgi?id=1418
This patch is going to fix a regression issue that is introduced
by commit b3497bad1221704a5dbc5da0b10f42625f1ad2ed.

Before commit b3497b, build launched a external GenFds.py to generate
Fd, so the global variable in GenFds.py was reset in each execution.

After commit b3497b, each GenFds run in the same python interpeter, so
we need to explicitly reset global variable in each GenFdsApi call.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>