initcalls: Add initial implementation

Create simple serial subsystem which makes use of initcalls, and convert
existing serial drivers to its use.

kernel/kernel.mk

@@ -7,6 +7,7 @@ OBJS_$(d) := $(d)/atags.o \
 	$(d)/console.o \
 	$(d)/font.o \
 	$(d)/framebuffer.o \
+	$(d)/init.o \
 	$(d)/kmalloc.o \
 	$(d)/list.o \
 	$(d)/math.o \