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>