Using relative path in a /boot/extlinux/extlinux.conf file fails to load
linux kernel.

Using a /boot/extlinux/extlinux.conf file:

  LABEL test
    LINUX ../linux/Image

Result in following error:

  Retrieving file: ../linux/Image
  Skipping test for failure retrieving kernel
  Boot failed (err=-14)

However, using sysboot cmd successfully load kernel using same file:

  sysboot mmc 1:1 any ${scriptaddr} /boot/extlinux/extlinux.conf

  Retrieving file: /boot/extlinux/../linux/Image

Fix relative path using bootmeth extlinux by supplying bootfile path
instead of subdir path in the call to pxe_setup_ctx, same as done in the
sysboot command.

Fixes: 31aefaf89a5b ("bootstd: Add an implementation of distro boot")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>

We use the terms 'distro' to mean extlinux but they are not really the
same. 'Distro' could refer to any method of booting a distribution,
whereas extlinux is a particular method.

Also we sometimes use syslinux, but it is better to use the same term in
all cases.

Rename distro to syslinux and also update bootstd uses of syslinux to use
extlinux instead.

Signed-off-by: Simon Glass <sjg@chromium.org>