certs: fix and refactor CONFIG_SYSTEM_BLACKLIST_HASH_LIST build

Commit addf4663 ("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 b8c96a6b
("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: addf4663 ("certs: Check that builtin blacklist hashes are valid")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Mickaël Salaün <mic@linux.microsoft.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>