The x86 OpenWrt target does not have device names, as x86 images are
generic for various hardware. Therefore, removing the target and
subtarget from the image name results in an indistiguishable image name.
Keep the target name for x86 images, so the resulting images are
distinguishable.
Fixes: #229
Signed-off-by: Fabian Bläse <fabian@blaese.de>