sstate: fetch .siginfo files from SSTATE_MIRROR

This would be useful for doing siginfo compares to understand why a build
is not reusing something when using SSTATE_MIRROR. No error will be reported
if it fails to find the .siginfo file

[YOCTO #2898]

[RP: Small tweaks]
(From OE-Core rev: 6d86690330f0d43839b904fced4b4b02cb27b8c6)

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/sstate.bbclass

@@ -216,6 +216,8 @@ def sstate_installpkg(ss, d):
 
     if not os.path.exists(sstatepkg):
         pstaging_fetch(sstatefetch, sstatepkg, d)
+    if not os.path.exists(sstagepkg + ".siginfo"):
+        pstaging_fetch(sstatefetch + ".siginfo", ssstatepkg + ".siginfo", d)
 
     if not os.path.isfile(sstatepkg):
         bb.note("Staging package %s does not exist" % sstatepkg)