For documentation purposes, the setup of java8 on server was not straight, the console-setup-alt.sh didn't work at first, to workaround it I needed to run the keytool manually:
keytool -genkey -keyalg rsa -alias idempiere -dname "CN=localhost.localdomain, OU=idempiere, O=idempiere, L=Krefeld, S=NW, C=DE" -keypass myPassword -validity 999 -keystore /opt/idempiere-server/jettyhome/etc/keystore -storepass myPassword
and then the console-setup-alt.sh was able to finish its job.
You cannot run java8 on top of a server installed with java7, you need to install it from scratch.
There is also a minor warning you can ignore
warning: ignoring option MaxPermSize=192m; support was removed in 8.0
Regards,