Move bootloader main to examples

This should remove some confusion around embassy-boot-* being a library
vs. a binary. The binary is now an example bootloader instead.

examples/boot/application/stm32l0/src/bin/b.rs

@@ -0,0 +1,24 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+#[cfg(feature = "defmt-rtt")]
+use defmt_rtt::*;
+use embassy::executor::Spawner;
+use embassy::time::{Duration, Timer};
+use embassy_stm32::gpio::{Level, Output, Speed};
+use embassy_stm32::Peripherals;
+use panic_reset as _;
+
+#[embassy::main]
+async fn main(_spawner: Spawner, p: Peripherals) {
+    let mut led = Output::new(p.PB6, Level::High, Speed::Low);
+
+    loop {
+        led.set_high();
+        Timer::after(Duration::from_millis(500)).await;
+
+        led.set_low();
+        Timer::after(Duration::from_millis(500)).await;
+    }
+}