http: Prepend prefix when the HTTP path is relative

http: Prepend prefix when the HTTP path is relative

There are two different HTTP drivers that can be used when requesting an
HTTP resource: the efi/http that uses the EFI_HTTP_PROTOCOL and the http
that uses GRUB's HTTP and TCP/IP implementation.

The efi/http driver appends a prefix that is defined in the variable
http_path, but the http driver doesn't.  So using this driver and
attempting to fetch a resource using a relative path fails.  Match the
behavior of efi/http.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Co-authored-by: Robbie Harwood <rharwood@redhat.com>
Signed-off-by: Robbie Harwood <rharwood@redhat.com>
Gbp-Pq: Topic network
Gbp-Pq: Name http-prepend-prefix-when-the-http-path-is-relative.patch