An rc file called rc.i2p has been placed into the /etc/rc.d directory. If you want to change installation dir, change the variable INSTALL_DIR on base-i2p.SlackBuild and rebuild the package. You also will need to do the same for the i2p package. The install script will insert everything needed into /etc/rc.d/rc.local and into /etc/rc.d/rc.local_shutdown automatically. If you want to start I2P at boot you have to chmod +x /etc/rc.d/rc.i2p