oeqa/sstate: Fix allarch samesigs test

The allarch sstate sigs test is supposed to compare the allarch sigs but
does not. Fix this.

Also rename the common function to make it clear it isn't just used by
the allarch test.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 549597d4)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>