bootutil: tweak state representation

Make num_sectors a size_t in struct boot_loader_state.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index 5990b80..9af54fb 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -1196,7 +1196,7 @@
 boot_go(struct boot_rsp *rsp)
 {
     int swap_type;
-    int slot;
+    size_t slot;
     int rc;
     int fa_id;