image-live.bbclass: remove MLPREFIX from syslinux

image-live.bbclass: remove MLPREFIX from syslinux

Fixed:
MACHINE = "qemux86-64"
require conf/multilib.conf
MULTILIBS = "multilib:lib32"
DEFAULTTUNE_virtclass-multilib-lib32 = "x86"
IMAGE_FSTYPES += "iso"

$ bitbake lib32-core-image-minimal
ERROR: lib32-core-image-minimal-1.0-r0 do_bootimg: The file /usr/include/printf.h is installed by both glibc and lib32-glibc, aborting

This was because:
lib32-syslinux -> lib32-glibc
virtual/kernel -> glibc

We can build 64bit syslinux (only build, not install) to fix the problem, the
do_bootimg only needs several data files of syslinux such as vesamenu.c32,
these files are not arch related.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>