Multiboot build system v4

Multiboot build system v4

In order to build the multiboot option rom, we need a Makefile and a tool
to sign the rom with.

Both are provided by this patch and mostly taken from the extboot source,
written by Anthony Liguori.

Once built, the resulting binary gets copied to pc-bios automatically.

Building also occurs automatically when on an x86 host.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>