Synchronize host and target for nc serial auto test ...

We're ensuring target and host start-up sync here in 2 ways:
1) adding a delay on host side to make sure the serial
initialization can happen before sending a character is sent to target
2) in case of serial_nc_rx_auto.py test, we're sending a
first character S which will trigger the move from rx+tx
to NC+rx.

This should  avoid any crossing case due to HSOT being faster than target or vice-versa

Handling empty strings better

Added tests that ensure correct handling of setting Serial pins to NC