Implements eval command and minor CLI cleanups.

Implemented eval command can be used to evaluate expressions.

The patch also documents echo command and allows to use log classes
instead of integer as a mask for echo.

sysdep/unix/log.c

@@ -70,6 +70,8 @@ static char log_buffer[LOG_BUFFER_SIZE];
 static char *log_buffer_pos;
 static int log_buffer_remains;
 
+const char *log_buffer_ptr = log_buffer;
+
 
 /**
  * log_reset - reset the log buffer