serial console even if the PROM uses bitmap console.
- Remove zs_hwflags since news68k does not have keyboard nor mouse at zs;
check zschan address set in zscninit() to detect console device.
- Use BPS_TO_TCONST() to set baudrate in zs_init_reg[].
- Remove zs_get_chan_addr(), which is not needed anymore.
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