Define a specific block device type for system so that we can
prevent raw writes to the system partition by anything other than
recovery.

Define a specific block device type for recovery so that we
can prevent raw writes to the recovery partition by anything
other than install_recovery or recovery.

These types must be assigned to specific block device nodes
via device-specific policy.  This change merely defines the types,
adds allow rules so that nothing will break when the types are assigned,
and adds neverallow rules to prevent adding further allow rules
on these types.

This change does not remove access to the generic block_device type
from any domain so nothing should break even on devices without these
type assignments.

Change-Id: Ie9c1f6d632f6e9e8cbba106f07f6b1979d2a3c4a
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>