kbuild: stop using config_filename in scripts/Makefile.modsign

Toward the goal of removing the config_filename macro, drop
the double-quotes and add $(srctree)/ prefix in an ad hoc way.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>

scripts/Makefile.modinst

@@ -66,9 +66,10 @@ endif
 # Don't stop modules_install even if we can't sign external modules.
 #
 ifeq ($(CONFIG_MODULE_SIG_ALL),y)
+CONFIG_MODULE_SIG_KEY := $(CONFIG_MODULE_SIG_KEY:"%"=%)
+sig-key := $(if $(wildcard $(CONFIG_MODULE_SIG_KEY)),,$(srctree)/)$(CONFIG_MODULE_SIG_KEY)
 quiet_cmd_sign = SIGN    $@
-$(eval $(call config_filename,MODULE_SIG_KEY))
-      cmd_sign = scripts/sign-file $(CONFIG_MODULE_SIG_HASH) $(MODULE_SIG_KEY_SRCPREFIX)$(CONFIG_MODULE_SIG_KEY) certs/signing_key.x509 $@ \
+      cmd_sign = scripts/sign-file $(CONFIG_MODULE_SIG_HASH) $(sig-key) certs/signing_key.x509 $@ \
                  $(if $(KBUILD_EXTMOD),|| true)
 else
 quiet_cmd_sign :=