angstrom-next: don't pin OLDEST_KERNEL

error caused in do_rootfs:

| NOTE: > Executing update_pixbuf_cache intercept ...
| FATAL: kernel too old
| WARNING: The postinstall intercept hook 'update_pixbuf_cache' failed (exit code: 127)! See log for details!

and many other 'kernel too old' messages.

So let's trust in OLDEST_KERNEL set by oe-core's bitbake.conf.

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>