The open source code for the paperwhite 3 stuff is here:
https://www.amazon.com/gp/help/custo...deId=200203720
I followed amazons directions with the cross compiler and built the linux package inside of it.
To get the USB storage drivers, I had to add the following to the .config that get created after running thier configuration command:
CONFIG_SCSI
CONFIG_BLK_DEV_SD
CONFIG_USB_STORAGE
I actually ran the amazon supplied build command first. Then added these to the .config and re-ran it, so I would get the names of the .ko kernel modules while watching the output of the build script, since it didn't re-build everything.