crypto: migrate ECDH(secp256r1) to X25519, drop TinyCrypt

- SC_PUBKEY_SIZE: 64 -> 32 bytes, SC_PUBKEY_ENC_SIZE: 72 -> 40
- sc_set_peer_public_key uses EVP_PKEY_derive with X25519
- sc_generate_keypair via EVP_PKEY_keygen(EVP_PKEY_X25519)
- sc_compute_public_key_from_private via EVP_PKEY_new_raw_private_key
- sc_validate_key: check non-zero (X25519 has no invalid keys)
- sc_obfuscate_pubkey: XOR now 32 bytes
- Remove all TinyCrypt code (~860 lines), always use OpenSSL
- Remove TinyCrypt from build system, drop test_crypto/test_ecc_encrypt
- Update all test keys (128 hex -> 64 hex)
- Fix bbr_integration.c split-line pubkeys

src/config_parser.h

@@ -79,7 +79,7 @@ struct CFG_CONTROL_ALLOW {
 };
 
 struct CFG_ALLOWED_KEY {
-    uint8_t key_bin[SC_PUBKEY_SIZE];  // public key in binary (64 bytes)
+    uint8_t key_bin[SC_PUBKEY_SIZE];  // public key in binary (32 bytes, X25519)
     struct CFG_ALLOWED_KEY *next;
 };