Adapt the example programs

Adapt the example programs if MBEDTLS_ENTROPY_HARDWARE_ALT
is defined.
diff --git a/programs/test/zeroize.c b/programs/test/zeroize.c
index 29cc0ac..6e7db4e 100644
--- a/programs/test/zeroize.c
+++ b/programs/test/zeroize.c
@@ -59,6 +59,19 @@
     mbedtls_printf( "       zeroize <FILE>\n" );
 }
 
+#if defined(MBEDTLS_ENTROPY_HARDWARE_ALT)
+int mbedtls_hardware_poll( void *data, unsigned char *output,
+                           size_t len, size_t *olen )
+{
+    size_t i;
+    (void) data;
+    for( i = 0; i < len; ++i )
+        output[i] = rand();
+    *olen = len;
+    return( 0 );
+}
+#endif
+
 int main( int argc, char** argv )
 {
     int exit_code = MBEDTLS_EXIT_FAILURE;