Fix NIMU Interface ID at build time

The Interface ID for a NIMU device can be suggested by the driver NIMU init function by setting a NETIF_DEVICE's index member to the desired ID and passing that device to NIMURegister(). This is only a suggestion though, and will not be used as the actual ID if another device is using the ID. 

This is a problem as many NDK settings/services need to know the Interface ID of a NIMU device AND this needs to be known before the stack is started. 

To fix this we should make it so that the index of a NIMU device in the NIMU Device Table is also the device's eventual Interface ID.