x86: add multiboot2 protocol support

x86: add multiboot2 protocol support

Add multiboot2 protocol support. Alter min memory limit handling as we
now may not find it from either multiboot (v1) or multiboot2.

This way we are laying the foundation for EFI + GRUB2 + Xen development.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Doug Goldstein <cardoe@cardoe.com>