[Paciente8159/uCNC PR#70] new settings extra commands

-new set of settings commands to control EEPROM/Flash storing (optional build setting ENABLESETTINGEXTRACMDS in cncconfig.h)

This set of new commands allow a more granular control over the settings stored in EEPROM/Flash to prevent wearing.
When enabled all grbl $x=val are only changed in SRAM.
3 additional commands are added:
$SS – Settings save. This stores all values to EEPROM/Flash
$SL – Settings load. This loads all values from EEPROM/Flash
$SR – Settings reset. This loads all default values from ROM