Convert download-prebuilt-firmware script to use new files

[mithro]

The following files are now generated as part of the travis build process;

• https://code.timvideos.us/HDMI2USB-firmware-prebuilt/revs.txt
• https://code.timvideos.us/HDMI2USB-firmware-prebuilt/index.txt
• https://code.timvideos.us/HDMI2USB-firmware-prebuilt/atlys/hdmi2usb/lm32/channels.txt
• https://code.timvideos.us/HDMI2USB-firmware-prebuilt/atlys/hdmi2usb/lm32/unstable.sha256sum

You can find the script which generates them here.

They are built as part of travis here. Look for the following lines at the very end of the log file;

The command "$PWD/.travis/build.sh" exited with 0.
> $PWD/.travis/update-prebuilt-list.sh

You can expand out the $PWD/.travis/update-prebuilt-list.sh line by clicking the little arrow to see the output from the script.

[mithro]

These files should make it much easier to workout how to download the prebuilt firmware. We should change the code to use them.

[mithro]

@CarlFK - PTAL.