Implement and test mbedtls_ecp_write_key

mbedtls_ecp_write_key is a mirror function to mbedtls_ecp_read_key, which
writes a private key back into a byte buffer in the correct format.
This is a helpful convenience function, since the byte order is defined
differently between Montgomery and Weierstrass curves. Since this difference
is accounted for in mbedtls_ecp_read_key, it made sense to add
mbedtls_ecp_write_key for the purpose of abstracting this away such that
psa_export_key doesn't need to take byte order into account.

Signed-off-by: Steven Cooreman <steven.cooreman@silabs.com>

include/mbedtls/ecp.h

@@ -1146,6 +1146,27 @@ int mbedtls_ecp_gen_key( mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
  */
 int mbedtls_ecp_read_key( mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
                           const unsigned char *buf, size_t buflen );
+
+/**
+ * \brief           This function exports an elliptic curve private key.
+ *
+ * \param grp_id    The ECP group identifier.
+ * \param key       The private key.
+ * \param olen      The amount of bytes written into the output buffer.
+ * \param buf       The output buffer containing the binary representation of
+ *                  the key. (Big endian integer for Weierstrass curves, byte
+ *                  string for Montgomery curves.)
+ * \param buflen    The total length of the buffer in bytes.
+ *
+ * \return          \c 0 on success.
+ * \return          #MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL if key is larger than buffer.
+ * \return          #MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE if the operation for
+ *                  the group is not implemented.
+ * \return          Another negative error code on different kinds of failure.
+ */
+int mbedtls_ecp_write_key( mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
+                           size_t *olen, unsigned char *buf, size_t buflen );
+
 /**
  * \brief           This function checks that the keypair objects
  *                  \p pub and \p prv have the same group and the