syslinux: fix booting memtest86+ using pxelinux regression

syslinux: fix booting memtest86+ using pxelinux regression

After syslinux was updated from 4.07 to 6.01, booting memtest86+ using
pxelinux would fail:
PXELINUX 6.01 2013-07-04 Copyright (C) 1994-2013 H. Peter Anvin et al
Loading memtest86... ok
Booting kernel failed: invalid argument

This backports the necessary upstream patches to allow memtest86+ to
boot using pxelinux again.

[YOCTO #5501]

(From OE-Core rev: ed9ccb8622b347173602be8b2126324d4fdf54d8)

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>