Add mbedtls_ecp_read_key

The private keys used in ECDH differ in the case of Weierstrass and
Montgomery curves. They have different constraints, the former is based
on big endian, the latter little endian byte order. The fundamental
approach is different too:
- Weierstrass keys have to be in the right interval, otherwise they are
  rejected.
- Any byte array of the right size is a valid Montgomery key and it
  needs to be masked before interpreting it as a number.

Historically it was sufficient to use mbedtls_mpi_read_binary() to read
private keys, but as a preparation to improve support for Montgomery
curves we add mbedtls_ecp_read_key() to enable uniform treatment of EC
keys.

For the masking the `mbedtls_mpi_set_bit()` function is used. This is
suboptimal but seems to provide the best trade-off at this time.
Alternatives considered:
- Making a copy of the input buffer (less efficient)
- removing the `const` constraint from the input buffer (breaks the api
and makes it less user friendly)
- applying the mask directly to the limbs (violates the api between the
modules and creates and unwanted dependency)

include/mbedtls/ecp.h

@@ -1093,6 +1093,22 @@ int mbedtls_ecp_gen_key( mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
                          int (*f_rng)(void *, unsigned char *, size_t),
                          void *p_rng );
 
+/**
+ * \brief           This function reads an ECP key.
+ *
+ * \param grp_id    The ECP group identifier.
+ * \param key       The destination key.
+ * \param buf       The the buffer containing the binary representation of the
+ *                  key. (Big endian integer for Weierstrass curves, byte
+ *                  string for Montgomery curves.)
+ * \param buflen    The length of the buffer in bytes.
+ *
+ * \return          \c 0 on success.
+ * \return          An \c MBEDTLS_ERR_ECP_XXX or \c MBEDTLS_MPI_XXX error code
+ *                  on failure.
+ */
+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 checks that the keypair objects
  *                  \p pub and \p prv have the same group and the