mailbox_capsule: reorder SECRequest enum

The rootserver's cut-down driver does not implement GetBuiltins, so
shuffle that to the end of the enum for compatibility.

Change-Id: I9e00e596cbe2cecf0a485a1f3d4acd81aebe70b8

app/src/mailbox_client.rs

diff --git a/app/src/mailbox_client.rs b/app/src/mailbox_client.rs
index 3249218..3993ab2 100644
--- a/app/src/mailbox_client.rs
+++ b/app/src/mailbox_client.rs
@@ -13,11 +13,13 @@
 
 #[derive(Debug, Serialize, Deserialize)]
 pub enum SECRequest<'a> {
-    GetBuiltins,           // Get package names -> Vec(String)
     FindFile(&'a str),     // Find file by name -> (/*fid*/ u32, /*size*/ u32)
     GetFilePage(u32, u32), // Read data from fid at offset -> <attached page>
 
     Test(/*count*/ u32), // Scribble on count words of supplied page
+
+    // NB: must be last to match up with stripped down rootserver driver
+    GetBuiltins,           // Get package names -> Vec(String)
 }
 
 #[derive(Debug, Serialize, Deserialize)]