Updating the booloader on macOS 10.13.1

[stefantimm]

./build/build-openocd.sh fails:

+ tar -xaf /Users/.../GRiSP/grisp-software/build/src/openocd-0.10.0.tar.bz2
Usage:
  List:    tar -tf <archive-filename>`
  Extract: tar -xf <archive-filename>
  Create:  tar -cf <archive-filename> [filenames...]
  Help:    tar --help

[holzingk]

I guess the a flag is not supported on macOS. Should be replaced with z.

[c-mauderer]

I would agree with @nextl00p.
@stefantimm: Could you try whether just replacing that flag in build/build-openocd.sh works?