rootfs_deb.bbclass: fix alternatives handling

rootfs_deb.bbclass: fix alternatives handling

After fixing update-alternatives-native the command:
   rmdir ${IMAGE_ROOTFS}/var/dpkg/alternatives

fails, because the directory is not empty, and the command:
  ln -s ${libdir}/opkg/alternatives ${IMAGE_ROOTFS}/var/dpkg/alternatives

creates a dangling symlink, because we mkdir only the parent directory of
the target.

Fixes: mkdir -p the alternatives directory itself and move alternatives
definitions into it before rmdiring the original alternatives directory

Signed-off-by: Filip Zyzniewski <filip.zyzniewski@gmail.com>
Signed-off-by: Tom Rini <tom_rini@mentor.com>