Warning free build achieved, coreboot documentation updated

[Pavel Roskin]

Hello!

I have fixed the remaining warnings in the coreboot build, so now GRUB
builds without warnings for all 7 supported platforms in the default
configuration.  Let's keep it this way.

I was unable to compile coreboot-v3, but I succeeded at compiling
coreboot-v2 for qemu.  It's not trivial, so I had to make a script.  The
script has been posted on http://grub.enbug.org/CoreBoot

-- 
Regards,
Pavel Roskin