lib/vsprintf.c: implement printf() in terms of vprintf()

This saves some code, both in terms of #LOC and .text size, and it is
also the normal convention that foo(...) is implemented in terms of
vfoo().

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>