modules: don't break modules_install on external modules with no key.

The script still spits out an error ("Can't read private key") but we
don't break modules_install.
Reported-by: Bruno Wolff III <bruno@wolff.to>
Original-patch-by: Josh Boyer <jwboyer@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

scripts/Makefile.modinst

@@ -16,8 +16,9 @@ PHONY += $(modules)
 __modinst: $(modules)
 	@:
 
+# Don't stop modules_install if we can't sign external modules.
 quiet_cmd_modules_install = INSTALL $@
-      cmd_modules_install = mkdir -p $(2); cp $@ $(2) ; $(mod_strip_cmd) $(2)/$(notdir $@) ; $(mod_sign_cmd) $(2)/$(notdir $@)
+      cmd_modules_install = mkdir -p $(2); cp $@ $(2) ; $(mod_strip_cmd) $(2)/$(notdir $@) ; $(mod_sign_cmd) $(2)/$(notdir $@) $(patsubst %,|| true,$(KBUILD_EXTMOD))
 
 # Modules built outside the kernel source tree go into extra by default
 INSTALL_MOD_DIR ?= extra