be.shellcmd: Support cmd.xxx options in --be-opts

Allow to override the default shell commands used to start and stop
the machine with custom commands, as in

   --be-opts="cmd.request-power-on='/usr/bin/power.sh something'"

Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/devtest/os/be/shellcmd/Machine.py

@@ -19,6 +19,9 @@ class Machine(MachineBase): # export
             'request-power-on': "jw-switch-allesnix.sh on",
             'cleanup'         : None
         }
+        for key in self.__cmds.keys():
+            if 'cmd.' + key in env.be_opts.keys():
+                self.__cmds[key] = env.be_opts['cmd.' + key]
 
     async def __run(self, phase):
         cmd = None