Assert active and dfu have same erase size and copy in smaller chunks

The copy from active to dfu (and vice versa) is now done in smaller portions depending on aligned_buf, which now does not need to be erase_size big.

embassy-boot/boot/src/mem_flash.rs

@@ -47,7 +47,6 @@ impl<const SIZE: usize, const ERASE_SIZE: usize, const WRITE_SIZE: usize> Defaul
 impl<const SIZE: usize, const ERASE_SIZE: usize, const WRITE_SIZE: usize> Flash
     for MemFlash<SIZE, ERASE_SIZE, WRITE_SIZE>
 {
-    const BLOCK_SIZE: usize = ERASE_SIZE;
     const ERASE_VALUE: u8 = 0xFF;
 }