certs: fix and refactor CONFIG_SYSTEM_BLACKLIST_HASH_LIST build

Commit addf466389d9 ("certs: Check that builtin blacklist hashes are
valid") was applied 8 months after the submission.

In the meantime, the base code had been removed by commit b8c96a6b466c
("certs: simplify $(srctree)/ handling and remove config_filename
macro").

Fix the Makefile.

Create a local copy of $(CONFIG_SYSTEM_BLACKLIST_HASH_LIST). It is
included from certs/blacklist_hashes.c and also works as a timestamp.

Send error messages from check-blacklist-hashes.awk to stderr instead
of stdout.

Fixes: addf466389d9 ("certs: Check that builtin blacklist hashes are valid")
Signed-off-by: Masahiro Yamada <[email protected]>
Reviewed-by: Jarkko Sakkinen <[email protected]>
Reviewed-by: Mickaël Salaün <[email protected]>
Signed-off-by: Jarkko Sakkinen <[email protected]>
diff --git a/certs/.gitignore b/certs/.gitignore
index 56637ac..cec5465 100644
--- a/certs/.gitignore
+++ b/certs/.gitignore
@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0-only
-/blacklist_hashes_checked
+/blacklist_hash_list
 /extract-cert
 /x509_certificate_list
 /x509_revocation_list