bitbake: bitbake-diffsigs: allow specifying task & follow deps recursively

bitbake: bitbake-diffsigs: allow specifying task & follow deps recursively

Add the ability to compare the two most recent runs of a specified task,
and follow dependent hash changes recursively. This enables you to trace
back and find exactly why a task was re-run after the fact.

Note that this relies on the metadata providing a function, hooked in
as bb.siggen.find_siginfo, which allows searching in the appropriate
places to find signature data files.

(Bitbake rev: cc70181659c07e04c205e17832846acf1ff31d28)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>