MODSIGN: Add option to not sign modules during modules_install

To allow the builder to sign only a subset of modules, or to sign the
modules using a key that is not available on the build machine, add
CONFIG_MODULE_SIG_ALL. If this option is unset, no modules will be
signed during build. The default is 'y', to preserve the current
behavior.

Signed-off-by: Michal Marek <mmarek@suse.cz>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/Makefile b/Makefile
index 77ea707..8e9f9ef 100644
--- a/Makefile
+++ b/Makefile
@@ -719,7 +719,7 @@
 export mod_strip_cmd
 
 
-ifeq ($(CONFIG_MODULE_SIG),y)
+ifdef CONFIG_MODULE_SIG_ALL
 MODSECKEY = ./signing_key.priv
 MODPUBKEY = ./signing_key.x509
 export MODPUBKEY