This function should return -1 if there is no trailing integer in the
string. Instead it returns 0. Fix it by checking for this condition at the
start.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

To allow the various string to number conversion functions to be used
when using tiny-printf,split them out into their own file which gets
build regardless of what printf implementation is used.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>