docs: design: Update maximum image size when using swap-move

When using the swap-move strategy, the area allocated to the trailer no
more need to have a size that is a multiple of the sector size, since
the area not allocated to the trailer in the first sector holding
trailer data can now be used to store firmware data.

Signed-off-by: Thomas Altenbach <[email protected]>

Author: taltenbach

docs/design.md

@@ -296,23 +296,23 @@ This algorithm is designed so that the higher sector of the primary slot is
 used only for allowing sectors to move up. Therefore the most
 memory-size-effective slot layout is when the primary slot is larger than
 the secondary slot by exactly one sector plus the size of the swap status area,
-rounded up to the total size of the sectors it occupies,
 although same-sized slots are allowed as well.
 The algorithm is limited to support sectors of the same
 sector layout. All slot's sectors should be of the same size.
 
 When using this algorithm the maximum image size available for the application
 will be:
 ```
-maximum-image-size = (N-1) * slot-sector-size - image-trailer-sectors-size
+maximum-image-size = (N-1) * slot-sector-size - image-trailer-size - fallback-trailer-padding
 ```
 
 Where:
   `N` is the number of sectors in the primary slot.
-  `image-trailer-sectors-size` is the size of the image trailer rounded up to
-  the total size of sectors its occupied. For instance if the image-trailer-size
-  is equal to 1056 B and the sector size is equal to 1024 B, then
-  `image-trailer-sectors-size` will be equal to 2048 B.
+  `image-trailer-size` is the size of the image trailer.
+  `fallback-trailer-padding` is the padding required for the fallback trailer and is equal to
+  `magic-aligned-size - (image-trailer-size mod slot-sector-size)` if that value is strictly greater
+  than zero, to zero otherwise.
+  `magic-aligned-size` is the size of the trailer magic field, rounded up to `BOOT_MAX_ALIGN`.
 
 This does imply, if there is any doubt, that the primary slot will be exactly
 one sector larger than the secondary slot due to the swap sector alone. It is