gluon-setup-mode: add DECT button to enter setup mode

many AVM devices do not have RESET/WPS buttons. So use the otherwise unused DECT/PHONE button to boot the device into setup mode.

This patch allows to enter the setup-mode by pressing the phone button
(often labeled as DECT) in addition to WPS and reset button.

This patch is necessary to allow supporting boards without a WPS and reset
button (e.g. AVM FRITZ!Box 7312).