hyperkube doesn't start any manifest from /etc/kubernetes/manifests
thanks to @AntoineCotten the problem was easily resolved.
first, I downgraded hyperkube from v1.6.0-beta.0_coreos.0 to v1.5.3_coreos.0. then I noticed an error in the kubelet log that made me understand that I had a major typo in /opt/bin/host-rkt.
I had exec nsenter -m -u -i -n -p -t 1 -- /usr/bin/rkt "\$@" instead of exec nsenter -m -u -i -n -p -t 1 -- /usr/bin/rkt "$@".
I escaped the $ when trying to paste the command line arguments, which then.. didn't. so.. not using 1.6.0-beta0 for now, that's ok! and fixed the script. now everything works again. thanks