grub-mkimage and other utils documentation

[Andrey Borzenkov]

I started to clean --pubkey in docs, hit grub-mkimage reference, hit
"grub-install is just a shell script" ... it really needs cleanup. Is
grub-install terse description intentional? If not, I'm going to
actually document all utilities and all options not defined as hidden.
We probably need to mention common options between various utilities
as well.

Or should some tools be skipped? I'm fine to document them in
grub-devel though, but I'd like to have them documented /somewhere/.