boot/bootutil: added log on swap type used

Added LOG for informing which a swap type is performed.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/bootutil/src/swap_scratch.c b/boot/bootutil/src/swap_scratch.c
index 0979ff5..f3275c7 100644
--- a/boot/bootutil/src/swap_scratch.c
+++ b/boot/bootutil/src/swap_scratch.c
@@ -699,6 +699,8 @@
     last_sector_idx = 0;
     last_idx_secondary_slot = 0;
 
+    BOOT_LOG_INF("Starting swap using scratch algorithm.");
+
     /*
      * Knowing the size of the largest image between both slots, here we
      * find what is the last sector in the primary slot that needs swapping.