Makefile: move the cleaning of staging_dir/target* from dirclean to clean

Currently "make clean" only clears the build_dir/target*, but leaves
staging_dir/target* intact. "make clean" should also clean the
staging_dir/target* directories, as in the current situation some old
packages or libraries may be linked into the firmware from staging_dir
despite a "make clean".

The patch reorganises clean / dirclean functionality so that
* "make clean" also clears the staging_dir/target* in addition to
build_dir/target*.
* "make dirclean" clears toolchain and host(=tools) directories from both
build_dir and staging_dir

signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45973 3c298f89-4303-0410-b956-a3cf2f4a3e73