Cryptographic security for MAX_BOOT_LEVEL

Use a KDF to generate a key for each boot level, anchored in a key
which can only be used once per boot.

Bug: 176450483
Test: aosp/1577966: ensure key created at level 40 stops working at 41
Test: keystore2_test
Change-Id: I12530cd13cb176251c8a0b5431d53c0a7c1bc02d
diff --git a/keystore2/src/lib.rs b/keystore2/src/lib.rs
index 62dc16a..0916af1 100644
--- a/keystore2/src/lib.rs
+++ b/keystore2/src/lib.rs
@@ -18,6 +18,7 @@
 pub mod apc;
 pub mod async_task;
 pub mod authorization;
+pub mod boot_level_keys;
 pub mod database;
 pub mod ec_crypto;
 pub mod enforcements;