dtc: Support opting-out of installing dtc

dtc is only used for static analysis (producing warnings) of the
DeviceTree sources. This means that valid Zephyr firmware can sanely
be built without it.

For some users, for instance Windows users that are not permitted to
use Chocolatey, installing dtc is problematic and installing it is not
worth the DT warnings that it provides.

To make using Zephyr easier for these users we make using DTC
recommended and opt-out, instead of mandatory.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>