modsign: Allow password to be specified for signing key

We don't want this in the Kconfig since it might then get exposed in
/proc/config.gz. So make it a parameter to Kbuild instead. This also
means we don't have to jump through hoops to strip quotes from it, as
we would if it was a config option.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

Documentation/kbuild/kbuild.txt

@@ -174,6 +174,11 @@ The output directory is often set using "O=..." on the commandline.
 
 The value can be overridden in which case the default value is ignored.
 
+KBUILD_SIGN_PIN
+--------------------------------------------------
+This variable allows a passphrase or PIN to be passed to the sign-file
+utility when signing kernel modules, if the private key requires such.
+
 KBUILD_MODPOST_WARN
 --------------------------------------------------
 KBUILD_MODPOST_WARN can be set to avoid errors in case of undefined