Remove generic const expressions from embassy-boot

* Remove the need for generic const expressions and use buffers provided in the flash config.
* Extend embedded-storage traits to simplify generics.
* Document all public APIs
* Add toplevel README
* Expose AlignedBuffer type for convenience.
* Update examples

examples/boot/application/nrf/src/bin/a.rs

@@ -36,7 +36,8 @@ async fn main(_spawner: Spawner) {
                 updater.write_firmware(offset, &buf, &mut nvmc, 4096).await.unwrap();
                 offset += chunk.len();
             }
-            updater.update(&mut nvmc).await.unwrap();
+            let mut magic = [0; 4];
+            updater.mark_updated(&mut nvmc, &mut magic).await.unwrap();
             led.set_high();
             cortex_m::peripheral::SCB::sys_reset();
         }