procd: switch buttons to the new "button" hotplug handler

This allows requesting "timeout" ACTION in script by returning a
positive value.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46580 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/system/procd/files/hotplug.json

@@ -66,7 +66,7 @@
 			[ "has", "BUTTON" ],
 			[ "eq", "SUBSYSTEM", "button" ],
 		],
-		[ "exec", "/etc/rc.button/%BUTTON%" ]
+		[ "button", "/etc/rc.button/%BUTTON%" ]
 	],
 	[ "if",
 		[ "eq", "SUBSYSTEM",