sstate.bbclass: fetch archive+siginfo in a single pstaging_fetch

sstate.bbclass: fetch archive+siginfo in a single pstaging_fetch

This avoids unnecessary duplication of setup. The only visible change in
behavior will be the case if siginfo exists and the archive does not, in which
case it'll redownload both, but this doesn't seem unresasonable to me,
particularly since the archive is downloaded first, making this case
particularly unlikely.

(From OE-Core rev: aa4991c307d4bbdd06c3cbf8448240b74c5e01c4)

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>