pxe: Move common parsing coding into pxe_util

Both the syslinux and pxe commands use essentially the same code to parse
and run extlinux.conf files. Move this into a common function.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Artem Lapkin <email2tema@gmail.com>
Tested-by: Artem Lapkin <email2tema@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>