Update the image's recorded URI when a download redirection occurs.
This ensures that URIs relative to a redirected download are resolved
correctly.

In particular, this allows for the use of relative URIs in scripts
that are themselves downloaded via a redirection, such as the HTTP 301
redirection used to fix up URIs pointing to directories but omitting
the trailing slash (e.g. "http://boot.ipxe.org/demo", which will be
redirected to "http://boot.ipxe.org/demo/").

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