Implements prunes_doubles function to removes doubles from a variables
mv extensions/libheader.* -> extensions/bin_libheader.*
Adds prefixes 'bin_' on C/C++ and assembly extensions
mv external_project_libheader.main_config_pre.mk -> bin_farincludes.main_config_pre.mk
Implements product_project_dir function geting a product's project directory
Lets extensions be able to add code when creating or importing a product
Implements product_c_cpp_rec_include_dirs function to recursively add all C/C++ inclusion directories
Removes old C/C++ include directories mechanism