The code called by do_rootfs explicitly needs the code obtained during
do_unpack. If built from sstate, it might not be present so we ensure
it is by adding an explicit dependency. This fixes build failures when
building from sstate.
(From OE-Core rev: 53d3c3caf1894e088ebf10fdf233cdf109b04da6)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>