kernel: Fix check_oldest_kernel

kernel: Fix check_oldest_kernel

The check_oldest_kernel() function requires utsrelease.h to be
generated. This file is generated during do_compile, so we need to delay
calling check_oldest_kernel() until after this.

With this change in place, I now see the expected warning when building
Linux 5.10.y.

Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>