buildtools-tarball: do not export OECORE_NATIVE_SYSROOT
When building a qemu image inside the environment created by the
buildtools-tarball, the qemu image cannot be started, as the runqemu
script uses the tunctl binary which cannot be found inside the sysroot
directory of the buildtools-tarball.
The buildtools-tarball is inherently a tool set instead of a fully
functional SDK, so leaving the OECORE_NATIVE_SYSROOT variable in the
environment will mess things up.
However, we do need a line of 'OECORE_NATIVE_SYSROOT=xxx' in the environment
setup script so that the SDK can be extracted and relocated correctly.
Thus, instead of exporting OECORE_NATIVE_SYSROOT, we use a comment here.
