multiboot2: make multiboot header optional

[Hollis Blanchard]

I would like to make the in-kernel multiboot header optional. If it's an
ELF file, we can load it, even if it doesn't have the header. We can
also pass the address of the multiboot tags to its entry point (it can't
hurt anything).

For kernels that need to communicate information to GRUB (e.g.
"vga_mode" from my previous email, or a.out load addresses), the
multiboot header would be needed for GRUB to locate the parameter area
within the executable.

-Hollis