toboot: fix WebUSB landing page descriptor request handler

[devanlai]

Implement the WebUSB landing page descriptor as a vendor request per the spec instead of via the standard GET_DESCRIPTOR request.

I finally got my tomu, so I was able to test this. This fixes issue #16. The target landing page, https://dfu.tomu.im is currently a 404, though.

[xobs]

I've cloned your webusb dfu page and put it up. Can you please see if that works?

Also, is there anything you would like me to do to distinguish your work on webusb?

[devanlai]

The page works in the sense that it writes the firmware successfully, but it does need a bit more polish:

• The final dfu GET_STATUS request fails during the manifestation step, which displays a giant error message. It's too early in the morning for me to remember if this is a quirk of my DFU implementation on the host side where it shouldn't expect GET_STATUS to work at that stage anyways, or if there's a DFU capability bit that should be set differently in the USB descriptor.
• My code doesn't check for dfu suffixes at all, so it just writes .dfu files it as if they were flat .bin files. Ideally there would be a simpler workflow where the dfu file is the main user input and it sets up the device filter by parsing the dfu suffix.

I'll try to sort out the first issue tonight. The second issue probably requires a bit more time than I have available this weekend.

[xobs]

The GET_STATUS may be an issue in the dfu sequence -- that was a part of the spec that I didn't really understand...

[devanlai]

I believe that the bmBitManifestationTolerant bit is supposed to control whether it transitions to MANIFEST_SYNC vs MANIFEST_WAIT_RESET, but in practice I probably shouldn't rely too much on those since they're phrased kind of ambiguously.

I've updated WebDFU to not complain about devices that enter MANIFEST_WAIT_RESET despite setting that bit.