bitbake: siggen: When printing signatures recursively, limit the output

bitbake: siggen: When printing signatures recursively, limit the output

Currently the code prints all differences. If the task dependencies have changed hash,
we recurse into those and print those differences as well. This leads to a lot
of output. The reality is if the parents changed signature, we might as well just
say that and recurse with no other output since we're much more interested in how
the parents changed in nearly all cases. The changes in the parent are probably
the same ones we'd have printed at each level anyway.

By doing this we focus the output more carefully on the thing the user wants/needs
to see.

(Bitbake rev: 7a17fd6e51a76d3582c357b79f5ef86e1969650c)

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