We have defined stdin, stdout and stderr as integer file descriptors,
but normally they should be FILE *. Also fprintf, fputc and fputs take
file descriptors instead of FILE *. As FILE * are inconvenient in the
barebox environment replace the f* functions with the corresponding d*
functions. dprintf is POSIX conform whereas dputc and dputs are barebox
specific, but do not conflict with any stdc function. fgetc is unused
and can be removed without replacing it.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>