If someone has changed TMPDIR in local.conf to a non-standard location, the
poky-qemu script currently doesn't handle this and assumes if BUILDDIR is set,
$BUILDDIR/tmp will exist.
Its simple to check if this exists and if not, to ask bitbake where the
directory is so this patch changes the code to do that.
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>