Suggested minor improvements the Mbed TLS configuration

[gilles-peskine-arm]

I'm an Mbed TLS maintainer. I've just had a look at how Mbed TLS is used in OpenThread as of 848de78. I'm not otherwise familiar with OpenThread. I noticed a few aspects where the OpenThread integration could be improved.

• As far as I can tell, only AES-128 is used, not other key sizes. You can save code size by defining MBEDTLS_AES_ONLY_128_BIT_KEY_LENGTH in mbedtls-config.h when MBEDTLS_VERSION_NUMBER >= 0x03050000.
• Only the encryption direction of AES is used. You can save code size by defining MBEDTLS_BLOCK_CIPHER_NO_DECRYPT in mbedtls-config.h when MBEDTLS_VERSION_NUMBER >= 0x03060000.
• HKDF could use mbedtls_hkdf_expand and mbedtls_hkdf_extract (available since before Mbed TLS 2.16) instead of reimplementing it on top of HMAC.
• In otPlatCryptoEcdsaGetPublicKey, since Mbed TLS 3.6, you can call mbedtls_ecp_write_public_key to get the public key in standard format (0x04 then fixed-size x and y).