GnuGenBootSector: Don't truncate output file
GnuGenBootSector: Don't truncate output file

When using GnuGenBootSector on a file rather than a device,
the previous code would cause the file size to be reset.
This made it impossible to build a DUET image file in a plain
file system file.

git-svn-id: f3880c1fc5096ce2109c8aec392d8e07f619f417@2074 c2973edb-eda0-4c78-bc6a-9341b269661f

tianocore/buildtools-BaseTools,tianocore/buildtools-BaseTools,bitcrystal/buildtools-BaseTools,bitcrystal/buildtools-BaseTools,tianocore/buildtools-BaseTools,tianocore/buildtools-BaseTools,bitcrystal/buildtools-BaseTools,tianocore/buildtools-BaseTools,bitcrystal/buildtools-BaseTools,bitcrystal/buildtools-BaseTools