altos-avr: Completely replace the spi slave code

altos-avr: Completely replace the spi slave code

Turns out the AVR we're using sucks at doing SPI slave. To get it
running at a reasonable data rate, I had to completely gut the
'sensible' code and run everything from the ISR with interrupts
disabled.

Even with this, the maximum SPI clock rate is somewhere around
200kHz. That's due to the singled buffered nature of the transmit
queue, the amount of time available between finishing one byte and
starting the next is very very small.

Signed-off-by: Keith Packard <keithp@keithp.com>