toolchain-shar-extract.sh: show progress when extracting SDK
Tar has supported a --checkpoint option since version 1.15.91, so it
should be safe to use here to print dots showing that it's still doing
something (technically it's not really progress unless you know how many
dots it's going to print, which even it doesn't know at the start, but
it's better than nothing).
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
