Fix for gcc warnings

Fixes warnings treated as errors when upgrading gcc crate to 0.3.54.
Warnings are due to non-used variables and comparison between signed
and unsigned.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index 9a921d8..b5e5c4a 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -149,7 +149,7 @@
     struct boot_swap_state state_scratch;
     struct boot_swap_state state_slot0;
     int rc;
-    int i;
+    size_t i;
     uint8_t source;
 
     rc = boot_read_swap_state_by_id(FLASH_AREA_IMAGE_0, &state_slot0);
@@ -1068,7 +1068,7 @@
     uint32_t sz;
     int first_sector_idx;
     int last_sector_idx;
-    int swap_idx;
+    uint32_t swap_idx;
     struct image_header *hdr;
     uint32_t size;
     uint32_t copy_size;