Allow using separate page sizes for state and dfu

* Less generics on bootloader. Keep PAGE_SIZE as a common multiple of
DFU and ACTIVE page sizes.
* Document restriction
* Add unit tests for different page sizes

examples/boot/stm32wl/src/bin/a.rs

@@ -40,7 +40,7 @@ async fn main(_s: embassy::executor::Spawner, p: Peripherals) {
             .unwrap();
         offset += chunk.len();
     }
-    updater.mark_update(&mut flash).await.unwrap();
+    updater.update(&mut flash).await.unwrap();
     //defmt::info!("Marked as updated");
     led.set_low();
     cortex_m::peripheral::SCB::sys_reset();