sstatesig: Allow exclusion of the root directory for do_package

The package task references WORKDIR at it's top level and we can't
easily make the timestamp for that determnistic due to writes to files
there and in other subdirs. We could try and force it to a specific value
but it is easier to just remove it from the package task, we don't need
it there or care about it in this case.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>