boards/shields: Move wnc_m14a2a to conditional Kconfig

Put shield configuration Kconfig flags in new Kconfig.defconfig.
This way shield BLE subsystem can now be selected by application
using NETWORKING symbol, similarly to what is done when modem is
directly present on board.
Additionally, move board Kconfig files to a similar scheme where
shields related Kconfig flags are made available under shield,
board and NETWORKING symbols.

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