lldpd: Use /etc/os-release instead of /etc/openwrt_*

With the addition of /etc/os-release patching lldpd to use
/etc/openwrt_release and to have the initscript use
/etc/openwrt_release and/or /etc/openwrt_version becomes
unnecessary.

Signed-off-by: Daniel Dickinson <lede@daniel.thecshore.com>