There is no need to take src_len and dest_len arguments. The case of
reading-only with a single output byte (originally src_len=1, dest_len>1)
is now handled by using the output buffer as the input buffer, and using
memset to fill the output byte into this buffer. This simplifies the
implementations of the spi_transfer protocol function.
modpybhspi now does the needed multiplexing, calling out to modpybspi
(bitbanging SPI) for suitable peripheral ID's. modmachinespi (previous
multiplexer class) thus not needed and removed.
modpybhspi also updated to following standard SPI peripheral naming:
SPI0 is used for FlashROM and thus not supported so far. SPI1 is available
for users, and thus needs to be instantiated as:
spi = machine.SPI(1, ...)
