bootutil: Add debug for encrypted key writing

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/bootutil_misc.c b/boot/bootutil/src/bootutil_misc.c
index 64882d2..6c05eff 100644
--- a/boot/bootutil/src/bootutil_misc.c
+++ b/boot/bootutil/src/bootutil_misc.c
@@ -525,6 +525,9 @@
     int rc;
 
     off = boot_enc_key_off(fap, slot);
+    BOOT_LOG_DBG("writing enc_key; fa_id=%d off=0x%lx (0x%lx)",
+                 fap->fa_id, (unsigned long)off,
+                 (unsigned long)fap->fa_off + off);
     rc = flash_area_write(fap, off, enckey, BOOT_ENC_KEY_SIZE);
     if (rc != 0) {
         return BOOT_EFLASH;