selinux: generate flask headers during kernel build

Add a simple utility (scripts/selinux/genheaders) and invoke it to
generate the kernel-private class and permission indices in flask.h
and av_permissions.h automatically during the kernel build from the
security class mapping definitions in classmap.h.  Adding new kernel
classes and permissions can then be done just by adding them to classmap.h.

Signed-off-by:  Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>