mbedtls_hkdf_expand() and mbedtls_hkdf_extract() be not be exposed without a warning

[bmurray7]

The functions mbedtls_hkdf_expand() and mbedtls_hkdf_extract() should not be publicly exposed through the API without a warning that they are, when used individually, not sufficient to provide the same security guarantees as mbedtls_hkdf().

I think it would be best to make mbedtls_hkdf_expand() and mbedtls_hkdf_extract() static and remove them from hkdf.h. If this is not an option, the documentation should be updated to make it more explicit that the extract and expand functions are intermediates and are not intended to be used for key derivation unless the inputs can be shown to satisfy the conditions in Krawczyk.

[RonEld]

@bmurray7 Thank you for reporting this!
We will consider what is the best option to resolve this

[ciarmcom]

ARM Internal Ref: IOTSSL-2373