Also handle more dtb paths for devices using deviceinfo_bootimg_dtb_second

Factor out dtb finding into its own function and remove redundant error message that now never would show since find_dtb errors out itself if the dtb isn't found.

Closes #8 (closed) Fixes e6929b63

@Minecrell would you have time to test this? I tested it in a device chroot and it seemed to be fine, but I haven't tested actual booting.