cli: add flash-fast command

[mndza]

This PR adds a command to Apollo CLI (apollo flash-fast) for flash programming. This command first programs a gateware SPI bridge to the FPGA and then uses it to access the configuration SPI flash.

[martinling]

I'm not sure if it makes sense to review this while greatscottgadgets/luna#220 is still in draft, since the two go together.

In fact, perhaps that flash bridge gateware should live here in the Apollo repository rather than in the LUNA repo?

[mndza]

True, let's wait until that other PR is ready (I'm marking as draft to avoid confusion).

I would like to create a PR with the ApolloAdvertiser injection code first (should that go in the cynthion repo?). That's why greatscottgadgets/luna#220 is still a draft (it creates a manual instance of the advertiser).

I do not have a strong opinion on the location of the flash bridge gateware. If we decide the code belongs here, I'm willing to move it.

[martinling]

There's three parts as I see it:

• support for platform-specific hooks, which needs to be added to USBDevice in the LUNA repo.
• the hooks, which need to be added to the platform definitions in the Cynthion repo.
• the ApolloAdvertiser gateware, which I think should live in apollo_fpga.gateware and can be imported from there by all Apollo-based platforms that need it (which will include Cynthion, URTI and others).

[mndza]

Rebased.

• Changed bInterfaceSubclass to 0x01 to avoid conflict with Apollo stub interface.
• Added a final trigger reconfiguration step when closing the connection with the flash bridge. This makes flash-fast inconsistent with the flash command (which does not trigger a reconfiguration), but avoids keeping the Flash bridge enumerated.

[martinling]

I've tested this on cynthion-test and it's all working great, but I have a couple of comments where we may need to discuss what we want to do.

[martinling]

All good!