MODSIGN: Add modules_sign make target

If CONFIG_MODULE_SIG is set, and 'make modules_sign' is called then this
patch will cause the modules to get a signature appended.  The make target
is intended to be run after 'make modules_install', and will modify the
modules in-place in the installed location.  It can be used to produce
signed modules after they have been processed by distribution build
scripts.

Signed-off-by: Josh Boyer <jwboyer@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (minor typo fix)