adding a new command in cooker to compute and get task signatures
this commit also add the associated command and event needed to get the
signatures using tinfoil
(Bitbake rev: 05c15162de90c41dad67e37a95ec9fdb440a7864)
Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>