avahi: do not install S05avahi-setup.sh in systemd setup

The AVAHI_INSTALL_AUTOIPD was unconditionally installing
S05avahi-setup.sh to /etc/init.d/, even in systemd
configurations. Therefore, this commit:

 * Moves the installation of S05avahi-setup.sh to a separate variable
   called AVAHI_INSTALL_AUTOIPD_INIT_SYSV.

 * Makes sure to use 'install -D' and a full destination path when
   installing S05avahi-setup.sh.

 * Moves the definition of AVAHI_INSTALL_AUTOIPD inside the
   BR2_PACKAGE_AVAHI_AUTOIPD conditional, so that both the variable
   definition and its addition to AVAHI_POST_INSTALL_TARGET_HOOKS are
   enclosed in the conditional.

 * Moves the S50avahi-daemon installation from AVAHI_INSTALL_INIT_SYSV
   to AVAHI_INSTALL_DAEMON_INIT_SYSV.

 * Uses 'install -D' to install S50avahi-daemon.

 * Adds a AVAHI_INSTALL_INIT_SYSV definition which calls both
   AVAHI_INSTALL_AUTOIPD_INIT_SYSV and AVAHI_INSTALL_DAEMON_INIT_SYSV.

Note: in a systemd configuration, there is no provision to do what
S05avahi-setup.sh is doing for sysv init configurations. Maybe this is
something that our systemd people should have a look at.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>