Fix boot failure for PowerMac11,2 when ``auto-boot?'' is true.

For some machines like PowerMac11,2, Open Firmware does not correctly
initialize console-related variables, like font-adr and line#, when
``auto-boot?'' is true; -1 is returned instead of correct values.

Fall back to wsfont embedded in kernel in this case. Also, do not use
line# if it is negative.

(rin)