Incorrect attempt to use root-path

[angusj]

I suspect the bug crept in while trying to simplify some tests on root_path, in function "netboot(...)", in src/usr/autoboot.c. Result of git bisect:

Code:

53d2d9e3c37d6170341818a254e18d341ee15511 is the first bad commit
commit 53d2d9e3c37d6170341818a254e18d341ee15511
Author: Michael Brown <mcb30@ipxe.org>
Date:   Wed Aug 26 22:35:42 2015 +0100

    [uri] Generalise tftp_uri() to pxe_uri()
    
    Merge the functionality of parse_next_server_and_filename() and
    tftp_uri() into a single pxe_uri(), which takes a server address
    (IPv4/IPv6/none) and a filename, and produces a URI using the rule:
    
     - if the filename is a hierarchical absolute URI (i.e. includes a
       scheme such as "http://" or "tftp://") then use that URI and ignore
       the server address,
    
     - otherwise, if the server address is recognised (according to
       sa_family) then construct a TFTP URI based on the server address,
       port, and filename
    
     - otherwise fail.
    
    Signed-off-by: Michael Brown <mcb30@ipxe.org>

:040000 040000 049d01d87fd2bf3a79e50a272564a91d6cd33963 6d8fc518906fdb4f9ba419c4878742811de7ac55 Msrc