init.rc: Restore the /mnt/sdcard symlink

"You are in a maze of twisty little symlinks, all alike."

Restore the /mnt/sdcard symlink, for compatibility with older Android
apps. This symlink was suppose to have been removed in the Gingerbread
time frame, but lives on.

Note: The /mnt/sdcard symlink was originally created in device specific
*.rc files in the device/vendor/hardware/* directory. This change moves
the creation of the symlink into the common init.rc file.

Bug: 25801877
Bug: 28108983
Change-Id: I2f9bf71bddffadb587d7376dfdfc8a546c84ec28
diff --git a/rootdir/init.rc b/rootdir/init.rc
index a95eaa2..3e2f0a5 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -87,6 +87,7 @@
 
     # Symlink to keep legacy apps working in multi-user world
     symlink /storage/self/primary /sdcard
+    symlink /storage/self/primary /mnt/sdcard
     symlink /mnt/user/0/primary /mnt/runtime/default/self/primary
 
     # root memory control cgroup, used by lmkd