Fixes: c9036fa ("Serialize `grub-script` literally into image.json")

Soon it will both set the platform ID and configure the console.

mantle/platforms.md also has a bunch, but that's more than I want to
unwind right now.

Stop specifying console= kernel arguments by default, except on specific
arch/platform pairs where we know we need them.  Do the same with console
configuration in grub.cfg.  More info is in
coreos/fedora-coreos-tracker#567.

Read platforms.yaml from the config repo to determine what console
settings should be applied for each arch/platform.  Convert the table for
the current arch to JSON and copy it to /boot/coreos/platforms.json in
the image, since coreos-installer needs the same data when overriding the
platform ID at install time.  If platforms.yaml is missing, continue to
apply the previous defaults.

We do this in two places: create_disk.sh configures console settings for
metal and qemu images from platforms.yaml.  For other platforms,
gf-set-platform reads platforms.json from the image, undoes any
qemu-specific settings, and applies settings for the target platform.