Fixed compilation errors with mingw32.
Add possibility to statically link against libnfc.
Make it possible to find libusb even if it is not installed in win32.

CMake now uses LIBUSB environment variable if there is no installed libusb
on windows to check if there is a libusb available.
Add makefile for building libnfc for avr.

Also added avr SPI bus.
Add AVR SPI driver.
Fix compile errors.
Add Atmel Studio projects for building lib for AVR.
Reduce memory footprint.

Maximum length of device name and connection string can now be specified
at compile time to reduce the amount of wasted space - 1 kbyte array is
a significant chunk of memory when you only have 8 kbytes available.
Fix bug in avr_spi_open.
Add "implementations" for err and getenv.

On small devices where there is no OS, there are no environment variables.
Merge https://code.google.com/p/libnfc into avr_port
AVR SPI chip data is set correctly in open.
Add framing to the PN53x commands.
Improve communication.
Start adding support for P70_IRQ.

This interrupt handling still requires some work.
Improve SPI device selection.
Fix typos in comments.
Make sure that P70_IRQ is enabled after SAM configuration.

P70_IRQ allows for polling PN532 readiness without occupying the SPI bus.
Deframe received data.

pn53x_avr_spi_receive now removes all framing around the response and checks the
length and data checksums.
Debugging.