multiboot2: Set min address for mbi allocation to 0x1000

multiboot2: Set min address for mbi allocation to 0x1000

In some cases GRUB2 allocates multiboot2 structure at 0 address, that is
a confusing behavior. Consumers of that structure can have internal NULL-checks
that will throw an error when get a pointer to data allocated at address 0.
To prevent that, define min address for mbi allocation on x86 and x86_64
platforms.

Signed-off-by: Lukasz Hawrylko <lukasz.hawrylko@linux.intel.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>