MODSERIAL with support for more devices

Dependents:   1D-Pong BMT-K9_encoder BMT-K9-Regelaar programma_filter ... more

This pull request has been accepted and merged in by Erik -

Update for latest version of mbed

Replaces _serial.index with _serial.serial.index if DEVICE_SERIAL_ASYNCH is #defined.

MODSERIAL with support for more devices Buffered, MODSERIAL, Serial