sstate: Allow validation of sstate singatures against list of keys

Allow a user to validate sstate objects against a list of keys, instead
of just any known key in the user's keychain.

(From OE-Core rev: 52ba0c5e6e2e3d5d01dc3f01404f0ab1bb29b3b5)

Signed-off-by: Daniel McGregor <daniel.mcgregor@vecima.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/sstate.bbclass

@@ -116,6 +116,9 @@ SSTATE_SIG_KEY ?= ""
 SSTATE_SIG_PASSPHRASE ?= ""
 # Whether to verify the GnUPG signatures when extracting sstate archives
 SSTATE_VERIFY_SIG ?= "0"
+# List of signatures to consider valid.
+SSTATE_VALID_SIGS ??= ""
+SSTATE_VALID_SIGS[vardepvalue] = ""
 
 SSTATE_HASHEQUIV_METHOD ?= "oe.sstatesig.OEOuthashBasic"
 SSTATE_HASHEQUIV_METHOD[doc] = "The fully-qualified function used to calculate \
@@ -372,7 +375,7 @@ def sstate_installpkg(ss, d):
             bb.warn("No signature file for sstate package %s, skipping acceleration..." % sstatepkg)
             return False
         signer = get_signer(d, 'local')
-        if not signer.verify(sstatepkg + '.sig'):
+        if not signer.verify(sstatepkg + '.sig', d.getVar("SSTATE_VALID_SIGS")):
             bb.warn("Cannot verify signature on sstate package %s, skipping acceleration..." % sstatepkg)
             return False