system_r is required by the policy_module macro, which however will
be expanded as empty if the module is built into base.pp. system_r
is defined in the kernel.te, its definition should be moved to the
top of base.conf so that other modules copied earlier into base.conf
than kernel.te could reference system_r in their unconditional block
properly.
Signed-off-by: Harry Ciao <qingtao.cao@windriver.com>