[rkp] Restore the EC_Key from the remotely provisioned key blob

This cl builds EcKey from the decrypted remotely provisioned
key blob inside the service VM.

The restored EC_Key will be used to sign the new certificate to
be appended to the remotely provisioned cert chain using ECDSA.

An implementation of __memset_chk has been added because it is
needed by BoringSSL.

Bug: 241428146
Test: atest libbssl_avf_nostd.test rialto_test
Change-Id: I805c73efa309c01f55eb13a085dcca36f1e39f54
diff --git a/libs/bssl/src/lib.rs b/libs/bssl/src/lib.rs
index 709e8ad..de81368 100644
--- a/libs/bssl/src/lib.rs
+++ b/libs/bssl/src/lib.rs
@@ -20,6 +20,7 @@
 
 mod aead;
 mod cbb;
+mod cbs;
 mod digest;
 mod ec_key;
 mod err;
@@ -32,6 +33,7 @@
 
 pub use aead::{Aead, AeadContext, AES_GCM_NONCE_LENGTH};
 pub use cbb::CbbFixed;
+pub use cbs::Cbs;
 pub use digest::Digester;
 pub use ec_key::{EcKey, ZVec};
 pub use hkdf::hkdf;