External EMAC drivers are currently directly attaching to lwip_stack.c
via mbed_lwip_bringup et al. Restore the original prototypes to avoid
compatibility breakage.
In case of carrier lost, we would like to inform PPP data pump.
That involved setting up link status flag down semaphores.
mbed_lwip_bringup() and mbed_lwip_bringdown() had been Ethernet specific only.
We extend these routines to support PPP as well. Currently we support only one interface
at a time. However, future enhancement to multi interface support should be trivial.