MODSIGN: Extract the blob PKCS#7 signature verifier from module signing

Extract the function that drives the PKCS#7 signature verification given a
data blob and a PKCS#7 blob out from the module signing code and lump it with
the system keyring code as it's generic.  This makes it independent of module
config options and opens it to use by the firmware loader.

Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Ming Lei <ming.lei@canonical.com>
Cc: Seth Forshee <seth.forshee@canonical.com>
Cc: Kyle McMartin <kyle@kernel.org>

include/keys/system_keyring.h

@@ -28,4 +28,9 @@ static inline struct key *get_system_trusted_keyring(void)
 }
 #endif
 
+#ifdef CONFIG_SYSTEM_DATA_VERIFICATION
+extern int system_verify_data(const void *data, unsigned long len,
+			      const void *raw_pkcs7, size_t pkcs7_len);
+#endif
+
 #endif /* _KEYS_SYSTEM_KEYRING_H */