From https://github.com/vyos/live-initramfs/pull/1

So if one naively tries to boot vyos over PXE by copying the append line in the isolinux config and then adding fetch=http://foo/filesystem.squashfs, one gets a kernel panic, presumably because the shell exits or crashes.

After much debugging, it turns out that the code that deals with fetch= does not setup networking first, so it never succeeds.

This pull request fixes that.

I also noticed that tftp:// URLs to fetch= were broken, so I backported the fix.