kernel-install: add compat with 'installkernel'

If 'kernel-install' is called as 'installkernel' it will be compatible with the
syntax used by the kernel's build system.

This means it can be called by doing 'make install' in a kernel build
directory, if the correct symlink has been installed (which we don't do by
default yet).

[Edit harald@redhat.com: removed basename and use shift]