build-setup: Add target for swift

Swift is a Power system under the witherspoon meta layer.

Change-Id: I4413c5cbd13f28fbf913badaef1da036feafc229
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/build-setup.sh b/build-setup.sh
index b119185..344a6d8 100755
--- a/build-setup.sh
+++ b/build-setup.sh
@@ -121,6 +121,10 @@
   palmetto)
     LAYER_DIR="meta-ibm/meta-palmetto"
     ;;
+  swift)
+    LAYER_DIR="meta-ibm/meta-witherspoon"
+    MACHINE="swift"
+    ;;
   witherspoon)
     LAYER_DIR="meta-ibm/meta-witherspoon"
     ;;