Use recursive make

Renamed install/ -> puavo-install/ to avoid confusion with `make
install'.
bootserver: install ltsp-update-kernels

This file is based on ltsp-update-kernels script from ltsp-server
package. It does not have any dependencies and is the only bit we need
from gigantic ltsp-server package. From now on, we don't need that
giant anymore, puavo-ltsp-bootserver is enough.