packer_freebsd.json: Update provision script

Switch to root before running freebsd.sh

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I5a252d53447344bc87e487be60025c80242d6beb
diff --git a/packer_freebsd.json b/packer_freebsd.json
index d7588b6..06bc2dd 100644
--- a/packer_freebsd.json
+++ b/packer_freebsd.json
@@ -44,8 +44,12 @@
     },
     {
       "type": "shell",
+      "inline": [ "su" ]
+    },
+    {
+      "type": "shell",
       "script": "freebsd.sh",
-      "execute_command": "{{.Vars}} sudo -E sh -eux '{{.Path}}'"
+      "execute_command": "{{.Vars}} sh -eux '{{.Path}}'"
     }
   ]
 }