BaseTools/GenFds: unbreak Region.PadBuffer

BaseTools/GenFds: unbreak Region.PadBuffer

In its current form, Region.PadBuffer() fills every second byte with 0x20,
the default separator string of Python's string.join():

https://docs.python.org/2/library/string.html#string.join

This corrupts some firmware because (a) 0x20 never corresponds to any
ErasePolarity, (b) the PadData produced are actually longer than Size.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Fixes: bd907fb6386560e621112beca7b7d381d0003967
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>