Add multiboot2 support

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1695286
Reported by 'Anatol Pomozov' on 2017-06-02 :

multiboot2 is a recent specification that resolves some of the issues of
multiboot. Multiboot2 is supported by some tools already (e.g. grub).

It would be great if one can run OS with multiboot2 using '-kernel'
option, similar as it is done now with multiboot images.

Quick googling shows there is a Debian bug and patch that adds multiboot
support https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=621529 Would it
be possible to integrate it to QEMU upstream?