sandbox: Correct loss of early output in SPL

At present fputc() is used before the console is available, then write()
is used. These are not compatible. Since fputc() buffers internally it is
better to use the write(), so that a partial line is immediately
displayed.

This has a slight effect on performance, but we are already using write()
for the vast majority of the output with no obvious impacts.

Signed-off-by: Simon Glass <sjg@chromium.org>