AirSpy Firmware version v1.0.0-rc6

Modifications for rc6:

• Added support for bit-packing (packing ADC samples to 12bits instead of using 16bits, packing use 25% less USB bandwidth versus No Packing)
  (Thanks to MM6DOS for that amazing feature, thanks also to Pierre HB9FUF for the initial packing proof-of-concept)
• Some cleanup/refactor/optimizations (mainly USB Stack)
• MD5 for https://github.com/airspy/firmware/releases/tag/v1.0.0-rc6 (build on 2015-07-14):
  md5sum airspy_fw/airspy_rom_to_ram/airspy_rom_to_ram.bin
262bc03869d13ed9dbb5d80a9a2eca43 airspy_fw/airspy_rom_to_ram/airspy_rom_to_ram.bin
airspy_fw/airspy_rom_to_ram/airspy_rom_to_ram.bin Size 18 848 bytes

Note:
- This firmware support only 10MSPS & 2.5MSPS.
- Becareful if you build it yourself the `date` of the day is included in `airspy_rom_to_ram.bin`, 
so if you want to have same MD5 you need to change your PC date to 2015-07-14 just before to build it.

• To flash the latest firmware in attachment airspy_fw_v1.0.0-rc6.zip
• For Windows:
  • 1. Extract the archive
  • 2. Connect AirSpy with USB to your computer
  • 3. Launch airspy_spiflash.bat from extracted archive directory
  • 4. Disconnect AirSpy from USB computer
  • 5. Connect AirSpy with USB to your computer, now you can use your favorite software

Note:
The new bit-packing option is supported since v1.0.6 host tools & airspy library see
https://github.com/airspy/host/releases/tag/v1.0.6