discover/pxe: Fix prefix autodetection when bootfile is in the root dir

discover/pxe: Fix prefix autodetection when bootfile is in the root dir

Currently, if the bootfile doesn't contain a directory, the path we use
for config file resolution will use the bootfile as the first component
of path.

For example, if bootfile is:

  pxelinux.0

the config files requested will be:

  pxelinux.0/<mac>
  pxelinux.0/<ips>
  pxelinux.0/default

For cases where bootfile is a single file, we need to use a blank
prefix.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>