Migrate sdcard0 to shell-accessible location.

Also remove mount() from adb, since it can come online long before
data partition is ready.  Set EXTERNAL_STORAGE environment variable
to point to owner for backwards compatibility.

Bug: 7005701
Change-Id: I63444f6636624eb7ad89f053daa289663424639e
diff --git a/rootdir/init.rc b/rootdir/init.rc
index 5a31a8e..1ed1323 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -56,6 +56,9 @@
     mkdir /cache 0770 system cache
     mkdir /config 0500 root root
 
+    # Directory for shell-visible mount points, like external storage
+    mkdir /mnt/shell 0700 shell shell
+
     # Directory for putting things only root should see.
     mkdir /mnt/secure 0700 root root