Add orion stanzas to various build system scripts. (ufs-community#38)

## DESCRIPTION OF CHANGES: 
* Add Orion stanzas to build scripts (and removed an old theia stanza).
* Changed "tag" to "branch" in Externals.cfg (which I think was a bug).

## TESTS CONDUCTED: 
Built on Orion successfully.

Externals.cfg

@@ -9,7 +9,7 @@ required = True
 [ufs_utils]
 protocol = git
 repo_url = https://github.com/NOAA-EMC/UFS_UTILS
-tag = develop
+branch = develop
 local_path = src/UFS_UTILS
 required = True
 

src/build_post.sh

@@ -17,6 +17,8 @@ if [ "$target" = "jet" ] ; then
   ./compile jet
 elif [ "$target" = "hera" ] ; then
   ./compile hera
+elif [ "$target" = "orion" ] ; then
+  ./compile orion
 elif [ "$target" = "wcoss_cray" ] ; then
   ./compile cray-intel
 elif [ "$target" = "wcoss_dell_p3" ] ; then

src/machine-setup.sh

@@ -27,9 +27,9 @@ if [ "$platform" = "no_platform_specified" ]; then
     elif [[ -d /scratch2/BMC/det ]] ; then
         # We are on NOAA Hera
         platform=hera
-    elif [[ -d /scratch3/BMC/det ]] ; then
-        # We are on NOAA Theia
-        platform=theia
+    elif [[ -d /work/noaa ]] ; then
+        # We are on MSU/NOAA Orion
+        platform=orion
     elif [[ -d /gpfs/hps && -e /etc/SuSE-release ]] ; then
         # We are on NOAA Luna or Surge
         platform=wcoss_cray