kbuild/mkspec: clean boot loader configuration on rpm removal

This patch add a rpm preuninstall scriptlet to cleanup the
boot loader configuration on kernel package uninstall.
The initrd for the to-be-removed kernel is deleted, too.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Michal Marek <mmarek@suse.com>