SPI: support SPI boards on Linux

This properly programs and verifies the eeprom on my SPI 7i90

typical commandline:
    mesaflash --device 7I90 --addr /dev/spidev1.0 --spi \
	--verify 7i90_spi_svst4_8.bit
Merge pull request #3 from jepler/spi

Spi support