[libc][stdio] cleanup printf definition and clients

Move implementation to stdio.c to sit next to the other
stdio definitions that use the debug output functions. Keep
the declaration of it, though, in printf.h to sit next to
sprintf, etc.

Don't include printf.h directly in other places, rely on stdio.h
to properly pull in these functions.

Change-Id: I357cb04a5c78185b8fde908193b672326c2ee542
Signed-off-by: Dima Zavin <dima@android.com>

platform/armemu/debug.c

@@ -22,7 +22,7 @@
  */
 #include <stdarg.h>
 #include <reg.h>
-#include <printf.h>
+#include <stdio.h>
 #include <kernel/thread.h>
 #include <platform/armemu/memmap.h>
 #include <platform/debug.h>