kbuild: support 'make modules_sign' with CONFIG_MODULE_SIG_ALL=n

Commit d890f510 ("MODSIGN: Add modules_sign make target") introduced
'make modules_sign' to manually sign modules.

Some time later, commit d9d8d7ed ("MODSIGN: Add option to not sign
modules during modules_install") introduced CONFIG_MODULE_SIG_ALL.
If it was disabled, mod_sign_cmd was set to no-op ('true' command).
It affected not only 'make modules_install' but also 'make modules_sign'.
With CONFIG_MODULE_SIG_ALL=n, neither modules_install nor modules_sign
is able to sign modules.

Kbuild has kept that behavior, and nobody has complained about it, but
I think it is weird.

CONFIG_MODULE_SIG_ALL=n should turn off signing only for modules_install.
If users want to sign modules manually, modules_sign should be offered.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

scripts/Makefile.modinst

@@ -95,7 +95,6 @@ endif
 # Signing
 # Don't stop modules_install even if we can't sign external modules.
 #
-ifeq ($(CONFIG_MODULE_SIG_ALL),y)
 ifeq ($(filter pkcs11:%, $(CONFIG_MODULE_SIG_KEY)),)
 sig-key := $(if $(wildcard $(CONFIG_MODULE_SIG_KEY)),,$(srctree)/)$(CONFIG_MODULE_SIG_KEY)
 else
@@ -104,13 +103,15 @@ endif
 quiet_cmd_sign = SIGN    $@
       cmd_sign = scripts/sign-file $(CONFIG_MODULE_SIG_HASH) "$(sig-key)" certs/signing_key.x509 $@ \
                  $(if $(KBUILD_EXTMOD),|| true)
-else
+
+ifeq ($(modules_sign_only),)
+
+# During modules_install, modules are signed only when CONFIG_MODULE_SIG_ALL=y.
+ifndef CONFIG_MODULE_SIG_ALL
 quiet_cmd_sign :=
       cmd_sign := :
 endif
 
-ifeq ($(modules_sign_only),)
-
 # Create necessary directories
 $(shell mkdir -p $(sort $(dir $(install-y))))