pyterm: make pin toggling optional

For the MSB-A2 the DTR and RTS pins have to be pulled down over the
serial interface in the beginning. (This is required because the Linux
usbserial driver pulls them up when initializing the device which set
the node into reset mode.) Since this is not necessary on most other
platforms and might even cause problems, it's better to make this an
optional behavior of pyterm.
Merge pull request #1702 from OlegHahm/pyterm_dtr_option

pyterm: make pin toggling optional