tiny-printf: Always print zeroes

tiny-printf: Always print zeroes

At present this does not print zero values in numeric format (hex and
decimal). Add a special case for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>