rsyslog: install init script and config file unconditionally

As discussed, users should use a rootfs overlay or a post-build script
instead of a custom skeleton to override files installed by Buildroot,
so there is no point in having conditions when installing init scripts
or configuration files.

Also, we take this opportunity to rename RSYSLOG_INSTALL_CONF_SCRIPT
to RSYSLOG_INSTALL_CONF because it is no longer installing an init
script.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>