rfkill: export persistent attribute in sysfs

This information allows userspace to implement a hybrid policy where
it can store the rfkill soft-blocked state in platform non-volatile
storage if available, and if not then file-based storage can be used.

Some users prefer platform non-volatile storage because of the behaviour
when dual-booting multiple versions of Linux, or if the rfkill setting
is changed in the BIOS setting screens, or if the BIOS responds to
wireless-toggle hotkeys itself before the relevant platform driver has
been loaded.

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/Documentation/rfkill.txt b/Documentation/rfkill.txt
index c8acd86..b486050 100644
--- a/Documentation/rfkill.txt
+++ b/Documentation/rfkill.txt
@@ -111,6 +111,8 @@
 
 	name: Name assigned by driver to this key (interface or driver name).
 	type: Driver type string ("wlan", "bluetooth", etc).
+	persistent: Whether the soft blocked state is initialised from
+	            non-volatile storage at startup.
 	state: Current state of the transmitter
 		0: RFKILL_STATE_SOFT_BLOCKED
 			transmitter is turned off by software