Makefile: fix compilation on current Raspbian Wheezy (include proper dbus directory)

See http://www.raspberrypi.org/forums/viewtopic.php?f=28&t=59548 for issue
Makefile: install COPYING and README into own directory

Previously the COPYING file ended up in /usr/share/doc/COPYING, same as the README file.
Obviously, this was wrong.
prepare-native-raspbian.sh: provide uninstall target in Makefile