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/
Make conditional with (pocl_get_bool_option("POCL_LEAVE_TEMP_DIRS", 0)). We want to retain the temporary files when debugging the kernel compiler and use this env to control this. The name is a bit misleading now, it should be now POCL_LEAVE_KERNEL_COMPILER_TEMP_FILES or similar.