boards/shields: x_nucleo_idb05a1: Move to conditional Kconfig

Put shield configuration Kconfig flags in new Kconfig.defconfig.
This way shield BLE susbystem can now be selected by application
using BT symbol, similarly to what is done when BlueNRG ship is
directly present on board.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>