Changes from ELETTRA:
Modifications in SerialLinux.cpp:
	-Add DevSerReadRetry() in SerialLinux.cpp
	    description:    
		     read a string from the serialline device in raw mode,
				 if first reading attempt successfull, retry to read "nretry" 
	       times; if no more data found exit on timeout without error.
		     Useful in case of reading long strings with no fixed lenght 
				 (ex. serialline hwfifo size < 64 bytes) 
				 Very unlucky case!!!
	       The maximum number of characters that can be read is
		     SL_MAXSTRING.

Modification in Serial.cpp:
	- Added dev_ser_read_retry()
	- Added retry method (SL_RETRY) to write_read(), dev_ser_read_char() and 
		dev_ser_read_string() 
  - write_read support for all readings modes (SL_NCHAR,SL_LINE,SL_RETRY)
	  (SL_RAW is equal to SL_RETRY with "nretry" set to 0) (see Serial.h)
