ECDH: Make the implementation use the new context
ECDH: Make the implementation use the new context

The functionality from public API functions are moved to
`xxx_internal()` functions. The public API functions are modified to do
basic parameter validation and dispatch the call to the right
implementation.

There is no intended change in behaviour when
`MBEDTLS_ECDH_LEGACY_CONTEXT` is enabled.

NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls