doc: update Makefile.include instructions
doc: update Makefile.include instructions

Specify the programmer using the PROGRAMMER variable, don't include programmer specific makefile and serial makefile

RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT,kYc0o/RIOT,miri64/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,OTAkeys/RIOT,jasonatran/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,miri64/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,miri64/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OTAkeys/RIOT,ant9000/RIOT,jasonatran/RIOT,miri64/RIOT,authmillenon/RIOT,kYc0o/RIOT,OTAkeys/RIOT,kaspar030/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,OTAkeys/RIOT,authmillenon/RIOT