Merge android-port & kernel-cache feature

Android port for merge

Major changes are in enabling kernel-cache
pocl_util.c : 
pocl_choose_dir() - chooses root folder for caching
pocl_create_source_dirs() - searches for matching program in sub-directories
other changes are either retaining files for caching or deleting unwanted files after cmpilation
use /sys/devices/system/cpu/possible file to find num-cores. Some android kernel shows no processor info on /proc/cpuinfo
plus pocl-android-sample in examples/
Does the driver_version include the repository revision for non-released versions? It would be useful. Otherwise we should add cache invalidate to 'make check' of pocl to force testing the under development kernel compiler instead of cached versions.