zephyr: Fix board logic

Don't set the board to qemu_x86 either; that's not right by any means.

Remove dts.overlay and build system logic related to it in
boot/zephyr/CMakeLists.txt. This file was a hack for arduino_101,
which is no longer supported by Zephyr.

Remove all the other DTC_OVERLAY_FILE logic in general while we're
here. Zephyr picks up
${APPLICATION_SOURCE_DIR}/boards/${BOARD}.overlay and uses it as
DTC_OVERLAY_FILE "for free" if DTC_OVERLAY_FILE is unset.

Adjust the minimum CMake version while we're here to account for
Zephyr build system changes.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
Signed-off-by: David Brown <david.brown@linaro.org>