initcalls: Add initial implementation

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

include/print.h

@@ -21,8 +21,8 @@
 #ifndef PRINT_H
 #define PRINT_H
 
-void print_init(void (*putc)(char));
+void print_init(int (*putc)(char));
 int print(char *fmt, ...);
-int print_func(void (putcf)(char), char *fmt, ...);
+int print_func(int (putcf)(char), char *fmt, ...);
 
 #endif /* PRINT_H */