port-amd64/50825: Kernel crash on Toshiba Chromebook 2

[johnnywhishbone%gmail.com@localhost]

>Number:         50825
>Category:       port-amd64
>Synopsis:       Kernel crash on Toshiba Chromebook 2
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Feb 17 12:30:00 +0000 2016
>Originator:     Ronny Schneider
>Release:        NetBSD-7.99.26-amd64-install
>Organization:
>Environment:
>Description:
Hello,

the latest NetBSD-HEAD (NetBSD-7.99.26-amd64-install) is not able to boot properly on the Toshiba Chromebook 2 (CB30-B-104). It's a Bay Trail device an has a flashed Coreboot/SeaBIOS 1.9.1

After a few seconds the boot process interrupts and starts the debugger:

Image 1: http://bilderhochladen.org/i/sE9tr8Dory/
Image 2: http://bilderhochladen.org/i/0nX5eSbF0/

Arch Linux ist running fine, so i have created a dmesg and lspci from there:

dmesg (Arch Linux): http://pastebin.com/FNEhYdsZ
lspci -v (Arch Linux): http://pastebin.com/pnFErXqG
Coreboot-log (cbmem): http://pastebin.com/asLqar5q

OpenBSD, FreeBSD and DragonFly are neither booting on this machine. Other Chromebooks have these problems too, e.g. the Chromebook Pixel.
Either the kernel is not able to find the usb stick to mount the root filesystem, or the kernel crashes in the boot process.

I reported a bug at DragonFlyBSD, FreeBSD and SeaBIOS:
- DragonFlyBSD: http://bugs.dragonflybsd.org/issues/2890
- FreeBSD: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207251
- SeaBIOS: http://www.seabios.org/pipermail/seabios/2016-February/010485.html

Booting via SD Card is supported but doesn't resolve the issue of the kernel not finding the boot device. It seems the xHCI-Controller is not supported.

Please let me know if i can provide more informations.
Thank you very much!
>How-To-Repeat:
Boot the NetBSD-7.99.26-amd64-install.img from an usb stick on this machine with standard options.
>Fix: