[yocto] Unset a variable

[Craig McQueen]

I'd like to make a distro config file, derived from conf/distro/poky.conf. But in my case, DISTRO_CODENAME is not really something that can contain a meaningful value, so I'd like to unset it. How can I unset the variable?

(This affects the format of VERSION in /etc/os-release. I guess alternatively I could modify it in a os-release.bbappend)

-- 
Craig McQueen