Recover from exception in serial.sendBreak() in linux.

When i execute the script workspace_tools/make.py to execute a test on my lpcxpresso1549 board, an exception is raised on line https://github.com/mbedmicro/mbed/blob/master/workspace_tools/make.py#L189 in serial.sendBreak().

Linux: Fedora 20 kernel 3.14.7-200.fc20.x86_64
Python: 2.7.5

Currently this is only tested in fedora 20 and with the lpcxpresso1549.
I can test it in a week on the LPC1768 board.

See mbedmicro/mbed#362