Add a /bin symlink for convenience.

We already have /etc and /sbin. As the Android world moves towards / being
on the system partition, the circumstances under which a /bin symlink
won't work are reduced. This should already be usable most of the time.

Bug: http://b/63142920
Test: `adb shell /bin/date`
Change-Id: I81c2209ae808ced186d05fbe1d5417ce8dd93ea7
diff --git a/rootdir/init.rc b/rootdir/init.rc
index 11b8383..2f91db5 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -48,8 +48,10 @@
     copy /proc/cmdline /dev/urandom
     copy /default.prop /dev/urandom
 
-    # Backward compatibility.
+    symlink /system/bin /bin
     symlink /system/etc /etc
+
+    # Backward compatibility.
     symlink /sys/kernel/debug /d
 
     # Link /vendor to /system/vendor for devices without a vendor partition.