sstate.bbclass: Fetch non-existing local .sig files if needed

For the case of a global shared state cache and a local sstate cache
with a mix of signed and un-signed entries, there is a case where
a .sig is missing locally, but may exist in the global sstate cache.

For this case, do not just fail sstate fetch, but rather backfill
the .sig file from the global sstate cache.

If this case is detected, re-run the fetch operation to get the
global .sig file (if it exists).

Signed-off-by: Tobias Hagelborn <tobias.hagelborn@axis.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>