Idea: move grub-emu to a separate platform

[Bean]

Hi,

Currently, the grub-emu is in every rmk files, but I don't see the
difference between grub-emu in i386-pc, i386-coreboot or i386-efi. In
fact, grub-emu runs inside the host os, it can't access firmware
facility anyway. One quick fix to move grub-emu to i386.rmk, but we
could also put it in a new platform altogether, something like
i386-host.

The advantage of using a separate platform is that we could now use
modules like the other platform, instead of embedding all source files
in grub-emu. For example, whenever we add a new command, we need to
modify the source list of grub-emu. But if we build i386-host like
other platform, we only need to change common.rmk.

What's more, i386-host is not just a testing tool, it could be a real
boot loader. kboot provides a minimum linux environment. We could use
i386-host as a front end, and use kexec to launch additional boot
files.

-- 
Bean