drivers: lora: Only create loramac-node library if not done before

The loramac-node library definition will also be created by
'subsys/lorawan' for the LoRaWAN support. Hence, just add the source
files if the previous declaration of the library is found else create
a new one.

Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>