syslinux: use cross toolchain to compile

syslinux: use cross toolchain to compile

syslinux is compling something with host gcc at do_install
stage, which leads to some unexpected errors with old gcc
on host.  Using our cross toolchain instead.

Signed-off-by: Lei Liu <lei.liu2@windriver.com>
Signed-off-by: Randy MacLeod <Randy.MacLeod@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>