Allow WIKSketch to store the device ID

1) modify LLAPSerial to restrict device IDs to the permitted range of
characters "-#@?\*ABCDEFGHIJKLMNOPQRSTUVWXYZ"
2) Add LLAP command aXXSAVE----- to WIKSketch to allow the device ID to
be saved to EEPROM. It is then read back from EEPROM at startup and used
to initialise LLAPSerial.