add datastorage module to handle data directory on various platform

On kindle, kobo and pocketbook the data directory is the current
running directory but on Android the app is installed in system
defined location and users may have no access to that location.
The same circumstances should be true for the upcoming Koreader for
Ubuntu touch, so the data directory (in which tessdata, dictionaries,
global settings, persistant defaults and probably history data are
stored) could be stored in another place.

platform/android/llapp_main.lua

@@ -1,6 +1,12 @@
 local A = require("android")
 A.dl.library_path = A.dl.library_path .. ":" .. A.dir .. "/libs"
 
+local ffi = require("ffi")
+ffi.cdef[[
+    char *getenv(const char *name);
+    int putenv(const char *envvar);
+]]
+
 -- check uri of the intent that starts this application
 local file = A.jni:context(A.app.activity.vm, function(JNI)
     local uri = JNI:callObjectMethod(
@@ -29,6 +35,9 @@ pcall(function() dofile("/sdcard/koreader/patch.lua") end)
 -- set proper permission for sdcv
 A.execute("chmod", "755", "./sdcv")
 
+-- set TESSDATA_PREFIX env var
+ffi.C.putenv("TESSDATA_PREFIX=/sdcard/koreader/data")
+
 -- create fake command-line arguments
 arg = {"-d", file or "/sdcard"}
 dofile(A.dir.."/reader.lua")