kernel-dirclean target:
 * do not delete all kernels, delete just the one corresponding to the current .config (avoids unnecessary kernel rebuilds in the "multiple builds within the same tree"-environments)
 * eliminate some code clones by introducing KERNEL_ID variable


git-svn-id: file:///var/svn/freetz/trunk@12019 149334a1-2f27-0410-a3b9-fc62619ac1e6