bootstd: Correct handling of script from network

When reading a script from a network, no block device is available.
Update the implementation to support this correctly, avoiding setting
environment variables which relate only to block devices.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
1 file changed