boot/grub2: use none platform when building for host

When creating the image we are using the target modules. Building
the modules for host is then unnecessary.

Lets configure host Grub2 for the 'none' platform.

Note that this still installs a platform-dependent file:
    .../host/lib/grub/i386-pc/config.h

This file does not seem to have much purpose, but it is harmless.
We did not care to provide a post-isntall hook to remove it.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>