classes/kernel: check OLDEST_KERNEL at configure time

classes/kernel: check OLDEST_KERNEL at configure time

If the kernel being built is older than OLDEST_KERNEL and we're building
with glibc, then the C library we're building is probably not going to
be compatible with the kernel and we should warn the user. (This is
easier to do here rather than when building glibc, because we don't
necessarily have the information we need to determine the kernel version
there, whereas we do here.)

Fixes [YOCTO #8653].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>