If ipxe fails to boot download kernel/initrd, it will give up and the
boot process will continue with the next boot device, and may end up
with a boot on the old disk image.
If ipxe has failed to download a file and after the pxe/ipxe_timeout
delay, it will now retry to download it instead of giving up.
This goto/retry pattern is common with IPXE. This is an example:
http://ipxe.org/cmd/autoboot
We can hardly implement complex solutions since we have a very limited
control over the ipxe version in use.
Closes-Bug: #1326656
Change-Id: I0fbb40c711a707ae9fae186e9afbe62b79168e28