nsfarm/mterm: fix invalid semicolon command in Shell

Originally mterm was using just semicolon but bare semicolon is invalid
in shell so it prints error. This instead passes decision of command to
be used to specific Cli implementations.

nsfarm/cli.py

@@ -118,12 +118,13 @@ class Cli(abc.ABC):
     def mterm(self, new_prompt=True):
         """Runs interactive terminal on this cli.
 
-        new_prompt controls if new semicolon commands should be automatically send to trigger print of new prompt in
-        terminal.
+        new_prompt controls if new command with no effect should be automatically send to trigger print of new prompt in
+        terminal. It is just something nice to have but it might not be desirable sometimes so it is possible to disable
+        it.
         """
         if new_prompt:
-            self._pe.sendline(";")
-            os.read(self._pe.fileno(), 2)  # Eat up the semicolon and new line character
+            self._pe.sendline(self._NOCMD)
+            os.read(self._pe.fileno(), len(self._NOCMD) + 1)  # Eat up no command and new line character
         mterm.mterm(self._pe.fileno())
 
 
@@ -133,6 +134,7 @@ class Shell(Cli):
     This is tested to handle busybox and bash.
     """
     _COLUMNS_NUM = 800
+    _NOCMD = ":"
     _SET_NSF_PROMPT = r"export PS1='nsfprompt:$(echo -n $?)\$ '"
     _NSF_PROMPT = r"(\r\n|\n\r|^)nsfprompt:([0-9]+)($|#) "
     _INITIAL_PROMPTS = [
@@ -194,6 +196,7 @@ class Shell(Cli):
 class Uboot(Cli):
     """U-boot prompt support class.
     """
+    _NOCMD = ";"
     _PROMPT = "(\r\n|\n\r|^)=> "
     _EXIT_CODE_ECHO = "echo $?"