Fix lockup on lpc11u35 when flow control is on
Fix lockup on lpc11u35 when flow control is on

If flow control is on on the lpc11u35 then the code can get stuck
indefinitely in uart reset waiting for all the bytes to get sent.
To prevent this, temporarily switch the UART to loopback mode
so it can be drained even if flow control is on.

sg-/DAPLink,google/DAPLink-port,google/DAPLink-port,google/DAPLink-port,google/DAPLink-port,sg-/DAPLink,sg-/DAPLink