Add possibility to run Miniterm on serial port of board

This is handy for debugging. It allows you to access serial console
during tests and that way developer can access directly state it
requires.
The way how it works is that developer is going to call:
board.serial_miniterm()
in appropriate location.

To make this work pytest have to be run without terminal redirection.
This is as easy as appending -s to pytest call.

nsfarm/board.py

@@ -2,9 +2,14 @@
 """
 import os
 import serial
+import serial.tools.miniterm
 from pexpect import fdpexpect
 from . import cli
 
+MINITERM_DEFAULT_EXIT = '\x1d'  # Ctrl+]
+MINITERM_DEFAULT_MENU = '\x14'  # Ctrl+T
+MINITERM_ENCODING = sys.getdefaultencoding()
+
 
 class Board():
     def __init__(self, target, target_config):
@@ -43,6 +48,29 @@ class Board():
         """
         return self.pexpect
 
+    def serial_miniterm(self):
+        """Runs interactive miniterm on serial TTY interface.
+
+        This can be used only if you disable capture in pytest (--capture=no).
+        """
+        miniterm = serial.tools.miniterm.Miniterm(self.serial)
+        miniterm.exit_character = MINITERM_DEFAULT_EXIT
+        miniterm.menu_character = MINITERM_DEFAULT_MENU
+        miniterm.set_rx_encoding(MINITERM_ENCODING)
+        miniterm.set_tx_encoding(MINITERM_ENCODING)
+
+        sys.stderr.write('\n')
+        sys.stderr.write('--- Miniterm on {p.name} ---\n'.format(p=miniterm.serial))
+        sys.stderr.write('--- Quit: {0} | Menu: {1} | Help: {1} followed by {2} ---\n'.format(
+            serial.tools.miniterm.key_description(miniterm.exit_character),
+            serial.tools.miniterm.key_description(miniterm.menu_character),
+            serial.tools.miniterm.key_description('\x08')))
+
+        miniterm.start()
+        miniterm.join()
+
+        sys.stderr.write("\n--- Miniterm exit ---\n")
+
 
 class Mox(Board):
     """Turris Mox boards.