certs: use $< and $@ to simplify the key generation rule

Do not repeat $(obj)/x509.genkey or $(obj)/signing_key.pem
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>

certs/Makefile

@@ -60,9 +60,8 @@ keytype-$(CONFIG_MODULE_SIG_KEY_TYPE_ECDSA) := -newkey ec -pkeyopt ec_paramgen_c
 
 quiet_cmd_gen_key = GENKEY  $@
       cmd_gen_key = openssl req -new -nodes -utf8 -$(CONFIG_MODULE_SIG_HASH) -days 36500 \
-		-batch -x509 -config $(obj)/x509.genkey \
-		-outform PEM -out $(obj)/signing_key.pem \
-		-keyout $(obj)/signing_key.pem $(keytype-y) 2>&1
+		-batch -x509 -config $< \
+		-outform PEM -out $@ -keyout $@ $(keytype-y) 2>&1
 
 $(obj)/signing_key.pem: $(obj)/x509.genkey FORCE
 	$(call if_changed,gen_key)