Add new bbclass for package feed signing

After this change signed package feeds should be enabled by adding
INERIT += "sign_package_feed"
instead of definining PACKAGE_FEED_SIGN="1".

(From OE-Core rev: 2ba901da9a07350cc8975fc951ef5054b32d421b)

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/sign_package_feed.bbclass

@@ -0,0 +1,24 @@
+# Class for signing package feeds
+#
+# Related configuration variables that will be used after this class is
+# iherited:
+# PACKAGE_FEED_PASSPHRASE_FILE
+#           Path to a file containing the passphrase of the signing key.
+# PACKAGE_FEED_GPG_NAME
+#           Name of the key to sign with. May be key id or key name.
+# GPG_BIN
+#           Optional variable for specifying the gpg binary/wrapper to use for
+#           signing.
+# GPG_PATH
+#           Optional variable for specifying the gnupg "home" directory:
+#
+inherit sanity
+
+PACKAGE_FEED_SIGN = '1'
+
+python () {
+    # Check sanity of configuration
+    for var in ('PACKAGE_FEED_GPG_NAME', 'PACKAGE_FEED_GPG_PASSPHRASE_FILE'):
+        if not d.getVar(var, True):
+            raise_sanity_error("You need to define %s in the config" % var, d)
+}