Add /dev/stdin, /dev/stdout, and /dev/stderr in recovery too.

Now these are in the main image, there's some chance of code assuming
they're present in recovery too (or command-line users using them out
of habit).

Bug: http://b/31824379
Test: builds
Change-Id: Ia19272cd1959685765099f3e15d1d1e63babd279

etc/init.rc

@@ -13,6 +13,10 @@ on init
     export ANDROID_DATA /data
     export EXTERNAL_STORAGE /sdcard
 
+    symlink /proc/self/fd/0 /dev/stdin
+    symlink /proc/self/fd/1 /dev/stdout
+    symlink /proc/self/fd/2 /dev/stderr
+
     symlink /system/bin /bin
     symlink /system/etc /etc