x86: fix upgrade by emptying SUPPORTED_DEVICES

x86: fix upgrade by emptying SUPPORTED_DEVICES

x86 uses append-metadata, but only for signing and not for the
metadata itself.

Since recently SUPPORTED_DEVICES was assigned with a global value
and is not empty anymore, append-metadata will now actually put
metadata into x86 images. This breaks sysupgrade on x86.

To fix it for the moment, let's just empty SUPPORTED_DEVICES for
this target again.

In the long term, one should either not add metadata to the images
if it is not desired, and/or remove the unintended fwtool check.

Fixes: f52081bcf938 ("treewide: provide global default for SUPPORTED_DEVICES")
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>