bootutil: Fix signed/unsigned comparison in boot_read_enc_key

When MCUBOOT_SWAP_SAVE_ENCTLV is enabled, a comparison between a signed
and an unsigned integer is made in boot_read_enc_key. This might cause a
warning to be emitted at compile-time.

Signed-off-by: Thomas Altenbach <thomas.altenbach@legrand.com>

boot/bootutil/src/bootutil_misc.c

@@ -268,7 +268,7 @@ boot_read_enc_key(const struct flash_area *fap, uint8_t slot, struct boot_status
 {
     uint32_t off;
 #if MCUBOOT_SWAP_SAVE_ENCTLV
-    int i;
+    uint32_t i;
 #endif
     int rc;