make croot slightly more useful

Have croot take an argument of a directory to end up in,
so we can jump around the tree even faster.

Change-Id: Ieb6a8d47fee0412006a11192b29ed44604961312
diff --git a/envsetup.sh b/envsetup.sh
index f050a9a..101ef13 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -922,7 +922,11 @@
 {
     T=$(gettop)
     if [ "$T" ]; then
-        \cd $(gettop)
+        if [ "$1" ]; then
+            \cd $(gettop)/$1
+        else
+            \cd $(gettop)
+        fi
     else
         echo "Couldn't locate the top of the tree.  Try setting TOP."
     fi