[IO] rework dprintf to take a debug level, defined in DEBUGLEVEL

printf at the moment just calls dprintf, but soon will become
a seperate I/O queue.
diff --git a/platform/at91sam7/debug.c b/platform/at91sam7/debug.c
index 2cca7b3..ad8f262 100644
--- a/platform/at91sam7/debug.c
+++ b/platform/at91sam7/debug.c
@@ -64,12 +64,12 @@
     }
 }
 
-void dputc(char c)
+void _dputc(char c)
 {
 	ser_putc(c);
 }
 
-void debug_halt()
+void platform_halt()
 {
 	arch_disable_ints();
     for(;;);