make: introduce $(CLEAN)
make: introduce $(CLEAN)

Currently, some targets are serialized before "clean" by conditionally
adding a dependency.

Make does allow ordering using "|" syntax in prerequisites, but for
"clean" that would always trigger the clean target.
By only conditionally setting $(CLEAN) to clean, that can be
circumvented.

The new CLEAN allows any recipe to be run after "clean" by specifying "|
$(CLEAN)" as prerequisite.

aeneby/RIOT,jasonatran/RIOT,OlegHahm/RIOT,x3ro/RIOT,rfuentess/RIOT,kaspar030/RIOT,aeneby/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,mtausig/RIOT,smlng/RIOT,toonst/RIOT,basilfx/RIOT,x3ro/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kYc0o/RIOT,rfuentess/RIOT,kYc0o/RIOT,yogo1212/RIOT,kaspar030/RIOT,miri64/RIOT,cladmi/RIOT,yogo1212/RIOT,toonst/RIOT,OlegHahm/RIOT,authmillenon/RIOT,smlng/RIOT,josephnoir/RIOT,toonst/RIOT,x3ro/RIOT,authmillenon/RIOT,cladmi/RIOT,yogo1212/RIOT,basilfx/RIOT,toonst/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,authmillenon/RIOT,OlegHahm/RIOT,authmillenon/RIOT,basilfx/RIOT,mtausig/RIOT,jasonatran/RIOT,yogo1212/RIOT,mtausig/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,cladmi/RIOT,rfuentess/RIOT,miri64/RIOT,josephnoir/RIOT,x3ro/RIOT,aeneby/RIOT,smlng/RIOT,mtausig/RIOT,ant9000/RIOT,kYc0o/RIOT,ant9000/RIOT,miri64/RIOT,kaspar030/RIOT,miri64/RIOT,kaspar030/RIOT,josephnoir/RIOT,OlegHahm/RIOT,basilfx/RIOT,yogo1212/RIOT,kaspar030/RIOT,aeneby/RIOT,jasonatran/RIOT,toonst/RIOT,jasonatran/RIOT,jasonatran/RIOT,authmillenon/RIOT,josephnoir/RIOT,miri64/RIOT,ant9000/RIOT,cladmi/RIOT,ant9000/RIOT,smlng/RIOT,josephnoir/RIOT,yogo1212/RIOT,rfuentess/RIOT,kYc0o/RIOT,mtausig/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,smlng/RIOT,x3ro/RIOT,cladmi/RIOT,ant9000/RIOT,aeneby/RIOT,basilfx/RIOT