OLDEST_KERNEL is passed into the glibc build in order to tune it for a particular minimum Linux kernel version. SDK_OLDEST_KERNEL is the equivalent variable for the SDK, but at the moment all that SDK_OLDEST_KERNEL does is influence a check at SDK installation time. In order to allow people to build the SDK for distributions that have older kernels, we need to ensure that OLDEST_KERNEL gets set to the value of SDK_OLDEST_KERNEL in the nativesdk namespace, i.e. by adding OLDEST_KERNEL = "${SDK_OLDEST_KERNEL}" in nativesdk.bbclass.