[cloud] Remove AWS public image access block automatically if needed

Making images public is blocked by default in new AWS regions.  Remove
this block automatically whenever creating a public image.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

contrib/cloud/aws-import

@@ -104,6 +104,7 @@ def import_image(region, name, family, architecture, image, public, overwrite,
     image_id = image['ImageId']
     client.get_waiter('image_available').wait(ImageIds=[image_id])
     if public:
+        client.disable_image_block_public_access()
         resource.Image(image_id).modify_attribute(Attribute='launchPermission',
                                                   OperationType='add',
                                                   UserGroups=['all'])