certs: simplify empty certs creation in certs/Makefile

To create an empty cert file, we need to pass "" to the extract-cert
tool, which is common for all the three call-sites of cmd_extract_certs.

Factor out the logic into extract-cert-in.

One exceptional case is PKCS#11 case, where we override extract-cert-in
with the URI.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>

certs/Makefile

@@ -13,12 +13,13 @@ obj-$(CONFIG_SYSTEM_BLACKLIST_KEYRING) += blacklist_nohashes.o
 endif
 
 quiet_cmd_extract_certs  = CERT    $@
-      cmd_extract_certs  = $(obj)/extract-cert $(2) $@
+      cmd_extract_certs  = $(obj)/extract-cert $(extract-cert-in) $@
+extract-cert-in = $(or $(filter-out $(obj)/extract-cert, $(real-prereqs)),"")
 
 $(obj)/system_certificates.o: $(obj)/x509_certificate_list
 
 $(obj)/x509_certificate_list: $(CONFIG_SYSTEM_TRUSTED_KEYS) $(obj)/extract-cert FORCE
-	$(call if_changed,extract_certs,$(if $(CONFIG_SYSTEM_TRUSTED_KEYS),$<,""))
+	$(call if_changed,extract_certs)
 
 targets += x509_certificate_list
 
@@ -52,22 +53,22 @@ $(obj)/x509.genkey:
 
 endif # CONFIG_MODULE_SIG_KEY
 
-# If CONFIG_MODULE_SIG_KEY isn't a PKCS#11 URI, depend on it
-ifneq ($(filter-out pkcs11:%, $(CONFIG_MODULE_SIG_KEY)),)
-X509_DEP := $(CONFIG_MODULE_SIG_KEY)
-endif
-
 $(obj)/system_certificates.o: $(obj)/signing_key.x509
 
-$(obj)/signing_key.x509: $(X509_DEP) $(obj)/extract-cert FORCE
-	$(call if_changed,extract_certs,$(if $(CONFIG_MODULE_SIG_KEY),$(if $(X509_DEP),$<,$(CONFIG_MODULE_SIG_KEY)),""))
+PKCS11_URI := $(filter pkcs11:%, $(CONFIG_MODULE_SIG_KEY))
+ifdef PKCS11_URI
+$(obj)/signing_key.x509: extract-cert-in := $(PKCS11_URI)
+endif
+
+$(obj)/signing_key.x509: $(filter-out $(PKCS11_URI),$(CONFIG_MODULE_SIG_KEY)) $(obj)/extract-cert FORCE
+	$(call if_changed,extract_certs)
 
 targets += signing_key.x509
 
 $(obj)/revocation_certificates.o: $(obj)/x509_revocation_list
 
 $(obj)/x509_revocation_list: $(CONFIG_SYSTEM_REVOCATION_KEYS) $(obj)/extract-cert FORCE
-	$(call if_changed,extract_certs,$(if $(CONFIG_SYSTEM_REVOCATION_KEYS),$<,""))
+	$(call if_changed,extract_certs)
 
 targets += x509_revocation_list