Install services for live images

Live images need more services installed, such as Hawaii.
installboot: Fix installation

Append systemd units to units_DATA so that Makefile-integration.am will
install them.
Fix systemd services links

Services are in the parent directory.
Use SYSTEMD_SYSTEM_UNIT_DIR instead of an hard-coded path

Install systemd units into SYSTEMD_SYSTEM_UNIT_DIR which is discovered
by configure, using pkg-config.