Hi,
As I've finally managed to get Fedora 25 booting from a USB attached HDD on a Raspberry Pi (2B) and I wasn't really able to it documented anywhere I'd like to document some of the things I found before I forget.
As the official Fedora support is so new, I haven't seen much discussion, so not really sure who to tell ... The Raspberry Pi page on the Fedora Wiki pointed me here.
The steps I used were very much the same as any of the guides for booting Raspbian from a USB HDD, but a couple of things I had to learn the hard way with Fedora:
1. The boot menu is extlinux and is configured from /boot/extlinux/extlinux.conf. This is where you need to set your root partition to point to the USB HDD partition. This may seem obvious, but considering there is cmdline.txt, /boot/grub/grub.conf and this to choose from it took a little time to figure.
2. The USB controller requires the dwc2 kernel module. This is not included in the default dracut image. It took me many hours to find this but once I rebuilt the image to include this module everything sprung to life.
For reference I used the following configuration:
/etc/dracut.conf.d/add-dwc2.conf
add_drivers+="dwc2"
I hope this is useful to someone and if anyone has suggestions for where to post it, fire away.
Lastly, it seems like a good idea to include the dwc2 module in dracut by default. Considering there are a bunch of other USB modules included (which I think can't do anything useful without the controller being initialised), it seems like an oversight not to include it. What would be the best place to request this?