openthread: Separate OpenThread Kconfigs from L2 layer

Moved OpenThread-related Kconfigs from L2 layer to
modules/openthread. All of those configs were not strictly related
to the L2 layer, so they fits better to OpenThread module.
This operation allows using OpenThread Kconfigs even if L2 layer is
disabled.

Enabling NET_L2_OPENTHREAD also configures those configs by
selecting the OPENTHREAD kconfig, so there is no change regarding
backwards compatibility. The only change is that the Kconfigs
related to Thread were moved to the modules space, so their
location in the menuconfig also changed.

Once it is done, a choice for setting different L2 implementations
seems to be redundant.

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