ipxe: update to newer commit

ipxe: update to newer commit

The current commit in tree is rather old. It has come to a point that
cherry-picking commits from upstream isn't trivial anymore.

There is long term plan to track ipxe upstream, but for 4.8 release, we
should just update ipxe to a newer commit (they are using rolling
release model now).

Forward-port the one boot prompt patch that is still relevant and retire
the rest which are already in upstream.

Reported-by: Juergen Schinker <ba1020@homie.homelinux.net>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>