o Disable ofw_console(4), sab(4) and zs(4).
sab(4) and zs(4) are disabled because the hardware controlled by
them is handled by uart(4)+puc(4) and the latter combination is
functionally complete and up to date.
ofw_console(4) is disabled because it doesn't claim the device it
controls (through OFW) and thus interferes with puc(4)+uart(4),
which has sufficient knowledge to extract the necessary information
from OFW to setup the console. Put differently, ofw_console(4) is
not a proper device driver and can only do harm. Its functionality
is completely handled by uart(4).
This commit makes uart(4) the default driver for serial ports.
MFC after: 2 weeks
d3477e8f82