lantiq/xrx200: make WLAN button responsive on Fritzbox 3370

lantiq/xrx200: make WLAN button responsive on Fritzbox 3370

Pressing the 'WLAN' button should enable/disable wireless activity.
However, on the Fritzbox 3370 this doesn't have an effect.
This patch changes the mapping of the physical WLAN button, so a button
press will emit an action for the 'rfkill' key instead of 'wlan'.
Apparently, this is what stock OpenWRT expects, and also what is
implemented for most other devices.

Signed-off-by: Dustin Gathmann <dzsoftware@posteo.org>
(cherry picked from commit a53bf63756ef2a266ae5f3b3507eeb7382b4cdc9)