Remove precompiled hexfiles - use demoboard for testing

Because users often are confused by the provide hexfiles and
the firmware from tinyUSBboard, remove the hexfile from further
repositories and therefore put a remark instead

If you want to try out USBaspLoader, use tinyUSBboard:

http://matrixstorm.com/avr/tinyusbboard/

However, for most recent version, you may need to compile your
own firmware.

Signed-off-by: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de>