net: openthread: Allow a platform to select a Settings backend.

Removed implying of NVS by the NET_L2_OPENTHREAD, and from now a
platform can choose between ZMS and NVS as a settings backend.
NET_L2_OPENTHREAD still requires NVS or ZMS backend so the config
depends on one of those.

Signed-off-by: Arkadiusz Balys <arkadiusz.balys@nordicsemi.no>