SYS: /B 00 not correctly accepted to enter a zero into boot unit variable

[ecm-pushbx]

This error is similar to #140 in that a zero value is read as meaning "uninitialised" but it could also be an explicitly zero value given by a user switch, /B 00 in this case.

Wrong use of the variable:

kernel/sys/sys.c

Lines 927 to 930 in b78abba

	/* if bios drive to store in boot sector not set and not floppy set to 1st hd */
	if (!opts->defBootDrive && (opts->dstDrive >= 2))
	opts->defBootDrive = 0x80;
	/* else opts->defBootDrive = 0x0; the 1st floppy */