AU: when generating payload, only delta generation needs to be in chroot

cros_generate_update_payload

Index: cros_generate_update_payload
diff --git a/cros_generate_update_payload b/cros_generate_update_payload
index ad63fd73d6eff3bd248bf8e26548e65017a7d801..0699f09f475cfc8b0a5a793105b0c9b7a10581f5 100755
--- a/cros_generate_update_payload
+++ b/cros_generate_update_payload
@@ -92,9 +92,6 @@ patch_kernel() {
   STATE_LOOP_DEV=""
 }
 
-# We should be in the chroot.
-assert_inside_chroot
-
 DEFINE_string image "" "The image that should be sent to clients."
 DEFINE_string src_image "" "Optional: a source image. If specified, this makes\
  a delta update."
@@ -109,6 +106,11 @@ eval set -- "${FLAGS_ARGV}"
 
 set -e
 
+if [ -n "$FLAGS_src_image" ]; then
+  # We need to be in the chroot for generating delta images
+  assert_inside_chroot
+fi
+
 locate_gpt
 
 DELTA=$FLAGS_TRUE