packer_freebsd.json: using "su root -c" instead of "sudo"

We don't have "sudo" pre-installed in the image.
So, using "su root -c" instead of "sudo" to run the script.

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: If77d290399fca882a54e0b4e11137b04489fb775
diff --git a/packer_freebsd.json b/packer_freebsd.json
index 06bc2dd..de32059 100644
--- a/packer_freebsd.json
+++ b/packer_freebsd.json
@@ -44,12 +44,8 @@
     },
     {
       "type": "shell",
-      "inline": [ "su" ]
-    },
-    {
-      "type": "shell",
       "script": "freebsd.sh",
-      "execute_command": "{{.Vars}} sh -eux '{{.Path}}'"
+      "execute_command": "{{.Vars}} su root -c '{{.Path}}'"
     }
   ]
 }