[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/makefile b/makefile
index a40bda0..fe533da 100644
--- a/makefile
+++ b/makefile
@@ -2,7 +2,7 @@
 include macros.mk
 
 PROJECT ?= beagle-test
-DEBUG ?= 1
+DEBUG ?= 2
 
 BUILDDIR := build-$(PROJECT)
 OUTBIN := $(BUILDDIR)/lk.bin