Move printer initialization to separate header

The RTEMS print user need to know nothing about a particular printer
implementation.  In particular get rid of the <stdio.h> include which
would be visible via <rtems.h>.

testsuites/samples/hello/init.c

@@ -11,6 +11,7 @@
 #include "config.h"
 #endif
 
+#include <rtems/printer.h>
 #include <rtems/test.h>
 
 #include <bsp.h> /* for device driver prototypes */