siggen: ensure nostamp tasks force dependent tasks to re-execute

siggen: ensure nostamp tasks force dependent tasks to re-execute

If a nostamp task is depended on by a non-nostamp task, then we want the
signature of that task to change such that it re-executes afterwards.
This is an unusual situation, but we want this to work in OE in
externalsrc.bbclass so that compilation happens every time it is
requested.

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