mksunxi_fit_atf.sh: Allow for this to complete when bl31.bin is missing

mksunxi_fit_atf.sh: Allow for this to complete when bl31.bin is missing

In situations like an autobuilder we are likely to not have bl31.bin
present and thus would fail to build and propagate the error upwards.
Instead, print a big warning to stderr so that human will see that
something is wrong but complete the build.

Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>