Initialize XDG_DATA_HOME and XDG_CACHE_HOME home environment variables

If XDG_CACHE_HOME and XDG_DATA_HOME are not set webkit doesn't know where it should store
it's data for cookies, local storage, caching etc. We're using /media/cryptofs/.sysmgr as
base path here as this was also used in legacy webOS.

Fixes #465

Signed-off-by: Simon Busch <morphis@gravedo.de>