update to buildroot guide

Yes, check it with "nios2-linux-uclibc-flthdr ~/rootfs/bin/busybox" .

cd ~/buildroot
make -C build_nios2/busybox clean
make#  prepare root fs
rm -rf ~/rootfs
cp -a ~/buildroot/build_nios2/root/* ~/rootfs
cd ~/rootfs#  remove include and lib to save space#  the dev nodes are generated via ~/download/rootfs_list
rm -rf dev usr/include usr/lib#  you may use this instead of the one from buildroot
cp ~/download/inittab etc