Rewrite ecdh_curve25519 program
Rewrite ecdh_curve25519 program

Rewrite the example ECDH x25519 program using the
high-level ECDH API.

Signed-off-by: Thomas Daubney <5b910c8e7c225fd62319de5b44eecef1f6356fe1@arm.com>

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