modsign: Allow external signing key to be specified

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>
diff --git a/Makefile b/Makefile
index dc87ec2..531dd16 100644
--- a/Makefile
+++ b/Makefile
@@ -870,7 +870,7 @@
 # export INITRD_COMPRESS := $(INITRD_COMPRESS-y)
 
 ifdef CONFIG_MODULE_SIG_ALL
-MODSECKEY = ./signing_key.priv
+MODSECKEY = $(CONFIG_MODULE_SIG_KEY)
 MODPUBKEY = ./signing_key.x509
 export MODPUBKEY
 mod_sign_cmd = scripts/sign-file $(CONFIG_MODULE_SIG_HASH) $(MODSECKEY) $(MODPUBKEY)