Top secrets sources NedoPC pentevo

Takes the image of main AVR firmware in IHEX form and
fixes CRC for the image in order for it to pass
bootloader CRC test.

It assumes the main program space is from 0x00000 to 0x1DFFF.

It also fills unused areas explicitly with 0xFFs, otherwise if the
AVR ISP programmer won't reprogram them, CRC check will fail.

It also checks whether last two bytes with CRC were unused in
an original image. If not, an error is generated.