Add little endian export to Bignum

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

Used `echo xx | tac -rs ..` to transform the test data to little endian.

tests/suites/test_suite_mpi.function

@@ -378,6 +378,37 @@ exit:
 }
 /* END_CASE */
 
+/* BEGIN_CASE */
+void mbedtls_mpi_write_binary_le( int radix_X, char * input_X,
+                                  data_t * input_A, int output_size,
+                                  int result )
+{
+    mbedtls_mpi X;
+    unsigned char buf[1000];
+    size_t buflen;
+
+    memset( buf, 0x00, 1000 );
+
+    mbedtls_mpi_init( &X );
+
+    TEST_ASSERT( mbedtls_mpi_read_string( &X, radix_X, input_X ) == 0 );
+
+    buflen = mbedtls_mpi_size( &X );
+    if( buflen > (size_t) output_size )
+        buflen = (size_t) output_size;
+
+    TEST_ASSERT( mbedtls_mpi_write_binary_le( &X, buf, buflen ) == result );
+    if( result == 0)
+    {
+
+        TEST_ASSERT( hexcmp( buf, input_A->x, buflen, input_A->len ) == 0 );
+    }
+
+exit:
+    mbedtls_mpi_free( &X );
+}
+/* END_CASE */
+
 /* BEGIN_CASE depends_on:MBEDTLS_FS_IO */
 void mbedtls_mpi_read_file( int radix_X, char * input_file,
                             data_t * input_A, int result )