nxp: combine MONOLITHIC_BT and MONOLITHIC_IEEE802154

Combine BLE and 802.15.4 monolithic build under a single config to make
it less error prone.
The choice between a BLE/802.15.4 combo firmware and a BLE only firmware
is done depending on the Soc (like RW610 vs RW612).

Signed-off-by: Axel Le Bourhis <axel.lebourhis@nxp.com>