[SPI] Make SPI_FLUSH() more versatile

Currently the SPI driver in core/ sets `SPI_FLUSH()` to be a single read
of the spi RX buffer. This is fine for many microcontrollers, but newer
platforms like the CC2538 have a multi-byte FIFO for the RX buffer. In
that case, a single read is not guaranteed to flush the RX. This commit
allows `SPI_FLUSH()` to be platform dependent if needed, but doesn't
change for platforms where it works.
[CC2538-SPI] Implement SPI_FLUSH, spi mode, !CS

Because the CC2538 has a multi-byte SPI RX FIFO, flushing the buffer
requires more than just a single read. This adds a loop that empties the
entire RX buffer on a FLUSH().

Different SPI chips needs different SPI settings. This commit adds a
function that allows chip drivers to configure the SPI peripheral before
using it.

The frame pin the driver was using as a chip select does not work as
most devices expect it to. It toggles after every byte, and most chips
interpret that as end of message. To make drivers more reliable, each
chip driver should setup a GPIO and assert it as needed.