[flashrom] Move PicKit2 form libusb0 to libusb

[tgotic]

Hi,
Recently I tried to use flashrom with PicKit2 on Windows.
I was unable to get it working with various libusb0 dll's,
so I rewrote pickit2_spi.c to use libusb (form libusb.info).
Since other external USB programmers use libusb,
flashrom doesn't depend on obsolete libusb0 anymore.

Please find attached new pickit2_spi.c, patch for Makefile, and a result 
of write operation.

Signed-off-by: Tomislav Gotić