multiboot2: kernel->GRUB communication

[Hollis Blanchard]

Right now the multiboot2 spec calls for bits to be set in the kernel
image to indicate to GRUB that various features are required or
requested.

I'm trying to come up with a cleaner way. I find all these bits very
confusing, actually, and I think it would be more clear if we had
something like this instead:

[... multiboot header ...]
.ascii "header_addr: 0xf00"
.ascii "load_addr: 0x0"
.ascii ...
(Actually, is it even worth supporting a.out at this point? This
particular example may be obsolete.)

Video mode information:
.ascii "vga_mode: text"
.ascii "columns: 80"
.ascii "rows: 24"

Thoughts?

-Hollis