os-release: Add VERSION_CODENAME
os-release: Add VERSION_CODENAME

Add VERSION_CODENAME to /etc/os-release and set it to
LAYERSERIES_COMPAT_meta-nilrt

Signed-off-by: Chaitanya Vadrevu <3811a3cb328197c46ad44d4be31bbd66ea52bfbb@ni.com>

ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt