idf_monitor: Small fixes (baud rate, EOL, /dev/tty.X on macOS, Ctrl-T on failure)

idf_monitor: Small fixes (baud rate, EOL, /dev/tty.X on macOS, Ctrl-T on failure)

* "make monitor" not passed the configured baud rate
  Closes #436 https://github.com/espressif/esp-idf/issues/436
* Pass toolchain prefix from sdkconfig into monitor tool
* Allow setting EOL in idf_monitor.py, use CRLF by default
* Detect if /dev/tty.X is used on macOS, warn and replace with /dev/cu.X
* If a build fails or gdb exits, ignore Ctrl-T (allowing Ctrl-T Ctrl-A/F to be same key sequence everywhere)
* Add a note about winpty on Windows
  Ref https://github.com/espressif/esp-idf/commit/02fdf8271de3a6db2ab9d4d6f023c160c84ebdbe#commitcomment-21369196