Update the SSID using sed in librarybox.sh

alter_SSID.py was a Python script to change the SSID in the /etc/conf/wireless file.  This has been replaced by a call to sed in librarybox.sh.

My LibraryBox (MR3020) is in the post so I haven't actually run this on a device but I believe sed is included in openwrt so I see no reason why this wouldn't work.  Obviously it needs to be tested before it is merged.  Functionally this should do exactly the same as the Python script.  It will change the SSID and create a backup called wireless_old