kernel.bbclass: fix pkg_postinst and pkg_postrm (from oe-core)

* Symptom: kernel symlink in /boot is not created.
* Rename in order to create the expected files:
* /var/lib/opkg/info/kernel-3.1.4.postinst
* /var/lib/opkg/info/kernel-3.1.4.postrm

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>