Makefile: fix copy of md5 sums for vmlinuz, initrd.pxe and health.pxe
install-server: ensure the role is updated
installs-server: upgrade system only for RHEL/CentOS

On Debian / Ubuntu, System is already upgraded by the packages & repo we
isntall. We only need to run the upgrade on RHEL / CentOS.
install-server: need glibc-static and unzip on RHEL7