init: increase read-ahead value to 256 kb

Apps and database file sizes have increased in recent Android versions,
which makes it practical to increase our read-ahead value from 128 kb.

General user experience seem to have improved a bit.

Change-Id: I35788444a8552a8f376c699ead163e951ed06b30

rootdir/init.target.rc

@@ -51,7 +51,8 @@ on boot
 
     write /proc/sys/vm/swappiness 100
 
-
+    # increase read-ahead value to 256 kb
+    write /sys/block/mmcblk0/queue/read_ahead_kb 256
 
 on post-fs-data
 # icd