It appears the ISP/SAF1761 sometimes is busy when reading the status
word of the PTD and simply returns a value of zero. Retry a few times
before giving up.

Sponsored by:	DARPA, AFRL
Add optional support for USB to BERI loader. Fix the linker script so
that the garbage collection feature can be used when linking.

Sponsored by:	DARPA, AFRL
Use own memory pool of 128K until further, hence that works the best.

Sponsored by:	DARPA, AFRL
Enable a login console on ttyu0 by default if it is a serial console. This
prevents surprising effects post-installation on serial-only x86 systems
(e.g. bhyve) where the kernel messages will appear, rc scripts will run,
and then there will be no login prompt and you will be locked out of the
system.

Thanks to Devin Teske and Peter Grehan for ideas and testing. With this
patch, it may also be possible to unify all or most of the machine-specific
ttys files.