Bluetooth: beacon: Add support for testing with nble driver

In addition to having a different CONF_FILE bt_enable needs to be given
a callback since nble driver cannot block while initializing.

Change-Id: I8bd2c79d0620e93dbecfef25c49c5529ff1ee954
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>