arch-atmega spi: buftx and bufrx 128bytes, allocated at runtime

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
arch-atmega/spi.c: handle char 0 as 'end of enquiry' when polling is incoming

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>