Documentation: ABI: overlays - global attributes

Documentation ABI entry for overlays sysfs entries.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

Author: pantoniou

Documentation/ABI/testing/sysfs-firmware-devicetree-overlays

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+What:		/sys/firmware/devicetree/overlays/
+Date:		October 2015
+Contact:	Pantelis Antoniou <pantelis.antoniou@konsulko.com>
+Description:
+		This directory contains the applied device tree overlays of
+		the running system, as directories of the overlay id.
+
+What:		/sys/firmware/devicetree/overlays/enable
+Date:		October 2015
+Contact:	Pantelis Antoniou <pantelis.antoniou@konsulko.com>
+Description:
+		The master enable switch, by default is 1, and when
+		set to 0 it cannot be re-enabled for security reasons.
+
+		The discussion about this switch takes place in:
+		http://comments.gmane.org/gmane.linux.drivers.devicetree/101871
+
+		Kees Cook:
+		"Coming from the perspective of drawing a bright line between
+		kernel and the root user (which tends to start with disabling
+		kernel module loading), I would say that there at least needs
+		to be a high-level one-way "off" switch for the interface so
+		that systems that have this interface can choose to turn it off
+		during initial boot, etc."