Fix EEPROM checksum to not be lastchar or lastchar+1

[drf5n]

Per gnea#158 the logical OR in the code below discards most of the checksum information, resulting in the final chekcsum being either the last character, or the last character +1.

grbl-Mega-5X/grbl/eeprom.c

Lines 130 to 149 in 7a293a9

	void memcpy_to_eeprom_with_checksum(unsigned int destination, char *source, unsigned int size) {
	unsigned char checksum = 0;
	for(; size > 0; size--) {
	checksum = (checksum << 1) || (checksum >> 7);
	checksum += *source;
	eeprom_put_char(destination++, *(source++));
	}
	eeprom_put_char(destination, checksum);
	}
	int memcpy_from_eeprom_with_checksum(char *destination, unsigned int source, unsigned int size) {
	unsigned char data, checksum = 0;
	for(; size > 0; size--) {
	data = eeprom_get_char(source++);
	checksum = (checksum << 1) || (checksum >> 7);
	checksum += data;
	*(destination++) = data;
	}
	return(checksum == eeprom_get_char(source));
	}

This fix brings the checksum in-line with grblHAL's implementation: https://github.com/grblHAL/core/blob/3a84b58d301f04279268b4ef1045fd6bc0961be5/nuts_bolts.c#L267-L278

See also the discussion at grbl#1249