Use ecp_gen_privkey() in ECDSA sign
Use ecp_gen_privkey() in ECDSA sign

Two different changes:

- the first one will allow us to store k in the restart context while
  restarting the following ecp_mul() operation

- the second one is an simplification, unrelated to restartability, made
  possible by the fact that ecp_gen_privkey() is now public

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