[boot] Scroll serial output before exit_boot_services

The last line of the boot output was always getting cut off by anything
else getting printed to the serial port. Adding two newlines to clear
the cursor of the previous output.

src/boot/status.cpp

@@ -147,6 +147,16 @@ status_line::do_fail(const wchar_t *message, uefi::status status)
     out->output_string(L"\r\n");
 }
 
+void
+status_line::do_blank()
+{
+    auto out = console::get().m_out;
+    int row = out->mode->cursor_row;
+
+    out->set_cursor_position(0, row);
+    out->output_string(L"\r\n\r\n");
+}
+
 
 
 status_bar::status_bar(video::screen *screen) :