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

rootdir/Android.mk

@@ -142,6 +142,7 @@ endif
 # create some directories (some are mount points) and symlinks
 LOCAL_POST_INSTALL_CMD := mkdir -p $(addprefix $(TARGET_ROOT_OUT)/, \
     sbin dev proc sys system data oem acct config storage mnt $(BOARD_ROOT_EXTRA_FOLDERS)); \
+    ln -sf /system/bin $(TARGET_ROOT_OUT)/bin; \
     ln -sf /system/etc $(TARGET_ROOT_OUT)/etc; \
     ln -sf /data/user_de/0/com.android.shell/files/bugreports $(TARGET_ROOT_OUT)/bugreports; \
     ln -sf /sys/kernel/debug $(TARGET_ROOT_OUT)/d; \

rootdir/init.rc

@@ -48,8 +48,10 @@ on init
     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.