bootutil: Add missing flash_area_close

boot_remove_image_from_flash has been missing flash_area_close.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index 27f2e5f..ee4f729 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -2814,6 +2814,7 @@
     rc = flash_area_open(area_id, &fap);
     if (rc == 0) {
         flash_area_erase(fap, 0, flash_area_get_size(fap));
+        flash_area_close(fap);
     }
 
     return rc;