commit 787f04bb6a - imx: add USB2_BOOT type

[Rasmus Villemoes]

Hi Peng

It seems that commit 787f04bb6a (imx: add USB2_BOOT type) broke our
board logic which relies on whether get_boot_device() returns USB_BOOT
or not. Instrumenting get_boot_device() shows that on our imx8mp board,
boot_instance is 3 when we're booting over USB, so boot_dev becomes 20
instead of 17 aka USB_BOOT.

I assume that for the boards/socs you mentioned in the commit message,
boot_instance could be 0 or 1. However, I don't see anywhere that uses
that USB2_BOOT define.

Short-term, I'll probably just revert 787f04bb6a locally. A bit longer
term, I'm thinking I should be doing rom_api_query_boot_infor() and
"decoding" the return value myself.

Is there any chance you could make some information on that ROM API
public so it's possible for outsiders to understand what's going on?

Rasmus