Sort the board index into alphabetical order.  (Note that we need to
sort alphabetically by the title text of each file, which isn't the
same ordering as sorting by the filename.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200507151819.28444-3-peter.maydell@linaro.org