boards: nrf52840dk_nrf52840: Add netif capability for a board

The `netif` capability is needed for certain samples to be built by the
CI for a target platform. As it was missing for nrf52840dk_nrf52840,
echo samples with OpenThread support were not being built and thus not
tested by the CI.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>