Add a UART example

It uses the legacy module, but some more work will be required to have a
high-level UART interface based on the ruduino::Register trait.

examples/uart.rs

@@ -0,0 +1,31 @@
+#![no_std]
+#![no_main]
+
+//! Serial port example.
+//!
+//! The output is viewable with simavr
+//!
+//! ```
+//! cargo build -Z build-std=core --target avr-atmega328p.json --examples --release
+//! simavr -m atmega328p target/avr-atmega328p/release/examples/uart.elf
+//! ```
+
+use ruduino::legacy::serial;
+
+#[no_mangle]
+fn main() {
+    const CPU_FREQUENCY_HZ: u64 = 16_000_000;
+    const BAUD: u64 = 9600;
+    const UBRR: u16 = (CPU_FREQUENCY_HZ / 16 / BAUD - 1) as u16;
+
+    serial::Serial::new(UBRR)
+        .character_size(serial::CharacterSize::EightBits)
+        .mode(serial::Mode::Asynchronous)
+        .parity(serial::Parity::Disabled)
+        .stop_bits(serial::StopBits::OneBit)
+        .configure();
+
+    for &b in b"Hello, from Rust!\n" {
+        serial::transmit(b);
+    }
+}