Prevent flashing non .bin files (and .bin files without gateware in them)

[mithro]

It is a common mistake to try to load a Xilinx .bit file rather than the .bin file. It would be good if the firmware detected this and prevented you from doing so.

Gateware .bin files should start with a header as described here;
https://github.com/timvideos/HDMI2USB-mode-switch/blob/master/hdmi2usb/modeswitch/files.py#L161-L170 - if that header is not detected when loading to SPI address 0, it is not going to work.

Even better would be to support decoding the .bit file on the fly and writing the generated .bin output into the flash. That is a lot more work however. (The .bit file is described at https://github.com/timvideos/HDMI2USB-mode-switch/blob/master/hdmi2usb/modeswitch/files.py#L66)