Add multiboot support.

Add multiboot support.

I've been successfully using SeaBIOS as secondary bootloader. In more details
I have GRUB2-as-payload in flash together with coreboot. SeaBIOS binary is on
the HDD and loaded by GRUB when needed. This has an unfortunate consequence
that I have to keep vga oprom in flash even if usually I boot without it. This
patches makes bios.bin.elf multiboot executable with files passed as modules.
Example:

menuentry "SeaBIOS (mb)" --unrestricted {
   root=ahci0,2
   multiboot /bios.bin.elf
   module /vgabios_x230.rom name=pci8086,0166.rom
}

the parameter name= specifies under which name SeaBIOS will see it.

Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>