certs: Fix build error when CONFIG_MODULE_SIG_KEY is empty
Since b8c96a6b46 ("certs: simplify $(srctree)/ handling and remove
config_filename macro"), when CONFIG_MODULE_SIG_KEY is empty,
signing_key.x509 fails to build:
CERT certs/signing_key.x509
Usage: extract-cert <source> <dest>
make[1]: *** [certs/Makefile:78: certs/signing_key.x509] Error 2
make: *** [Makefile:1831: certs] Error 2
Pass "" to the first argument of extract-cert to fix the build error.
Link: https://lore.kernel.org/linux-kbuild/20220120094606.2skuyb26yjlnu66q@lion.mk-sys.cz/T/#u
Fixes: b8c96a6b46 ("certs: simplify $(srctree)/ handling and remove config_filename macro")
Reported-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Michal Kubecek <mkubecek@suse.cz>
e6340b6526