PlatformPayloadFeaturePkg.py: Add Linux support

- Use OS specific path seperator instead of hardcoding ';'
- Fix case of build artifact FV/PLATFORMPAYLOAD.Fv

Test:
python Features/Intel/PlatformPayloadFeaturePkg/PlatformPayloadFeaturePkg.py
works on Linux.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=4358



Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Cc: Sai Chaganty <rangasai.v.chaganty@intel.com>
Cc: Isaac Oram <isaac.w.oram@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Isaac Oram <isaac.w.oram@intel.com>