refactor: replace manual debug_putchar with libkos::println!

Init, storage server, and hello app now use println!/print! macros
instead of manual byte-by-byte UART output.

crates/apps/hello/src/main.rs

@@ -5,5 +5,5 @@ extern crate libkos;
 
 #[unsafe(no_mangle)]
 extern "C" fn main() {
-    // The runtime handles _start, exit, and panic.
+    libkos::println!("Hello from k-os!");
 }