Reimplemented SW SPI for DUE in assembler. This allows to reach 12Mhz as SPI...

Reimplemented SW SPI for DUE in assembler. This allows to reach 12Mhz as SPI Clock and improves 4x the transfer speed to the SD card. This is REQUIRED so access to SD from USB is usable (allows 600Kbytes/second transfer speeds)