Add little endian import to Bignum

The function `mbedtls_mpi_read_binary()` expects big endian byte order,
but we need to be able to read from little endian in some caseses. (For
example when handling keys corresponding to Montgomery curves.)

Used `echo xx | tac -rs .. | tr [a-z] [A-Z]` to transform the test data
to little endian and `echo "ibase=16;xx" | bc` to convert to decimal.

include/mbedtls/bignum.h

@@ -490,8 +490,24 @@ int mbedtls_mpi_read_binary( mbedtls_mpi *X, const unsigned char *buf,
                              size_t buflen );
 
 /**
- * \brief          Export an MPI into unsigned big endian binary data
- *                 of fixed size.
+ * \brief          Import X from unsigned binary data, little endian
+ *
+ * \param X        The destination MPI. This must point to an initialized MPI.
+ * \param buf      The input buffer. This must be a readable buffer of length
+ *                 \p buflen Bytes.
+ * \param buflen   The length of the input buffer \p p in Bytes.
+ *
+ * \return         \c 0 if successful.
+ * \return         #MBEDTLS_ERR_MPI_ALLOC_FAILED if memory allocation failed.
+ * \return         Another negative error code on different kinds of failure.
+ */
+int mbedtls_mpi_read_binary_le( mbedtls_mpi *X,
+                                const unsigned char *buf, size_t buflen );
+
+/**
+ * \brief          Export X into unsigned binary data, big endian.
+ *                 Always fills the whole buffer, which will start with zeros
+ *                 if the number is smaller.
  *
  * \param X        The source MPI. This must point to an initialized MPI.
  * \param buf      The output buffer. This must be a writable buffer of length