Makefile.include: allow specifying makefiles to be parsed
Makefile.include: allow specifying makefiles to be parsed

This allows specifying a list of files that should be parsed by make at
the beginning and at the end of Makefile.include.

It is a generic mechanism to allow specifying system wide configuration:

* Globally overwrite the 'TERMPROG'
* Specify a hard written port / debug_adapter_id for some BOARD values
* Define you own specific targets
* Override default targets

It can include file before and after Makefile.include to allow handling
different configurations.

kaspar030/RIOT,gebart/RIOT,OlegHahm/RIOT,mtausig/RIOT,BytesGalore/RIOT,x3ro/RIOT,gebart/RIOT,toonst/RIOT,gebart/RIOT,toonst/RIOT,mfrey/RIOT,RIOT-OS/RIOT,gebart/RIOT,x3ro/RIOT,miri64/RIOT,x3ro/RIOT,OlegHahm/RIOT,authmillenon/RIOT,basilfx/RIOT,mfrey/RIOT,mfrey/RIOT,miri64/RIOT,OlegHahm/RIOT,basilfx/RIOT,cladmi/RIOT,josephnoir/RIOT,basilfx/RIOT,rfuentess/RIOT,OTAkeys/RIOT,aeneby/RIOT,smlng/RIOT,miri64/RIOT,OTAkeys/RIOT,smlng/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,miri64/RIOT,mfrey/RIOT,BytesGalore/RIOT,x3ro/RIOT,x3ro/RIOT,authmillenon/RIOT,yogo1212/RIOT,yogo1212/RIOT,rfuentess/RIOT,BytesGalore/RIOT,RIOT-OS/RIOT,gebart/RIOT,kaspar030/RIOT,josephnoir/RIOT,aeneby/RIOT,mtausig/RIOT,josephnoir/RIOT,A-Paul/RIOT,aeneby/RIOT,lazytech-org/RIOT,OTAkeys/RIOT,rfuentess/RIOT,lazytech-org/RIOT,josephnoir/RIOT,cladmi/RIOT,yogo1212/RIOT,ant9000/RIOT,jasonatran/RIOT,cladmi/RIOT,authmillenon/RIOT,aeneby/RIOT,authmillenon/RIOT,jasonatran/RIOT,rfuentess/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,cladmi/RIOT,authmillenon/RIOT,lazytech-org/RIOT,kYc0o/RIOT,kYc0o/RIOT,smlng/RIOT,kYc0o/RIOT,kaspar030/RIOT,A-Paul/RIOT,cladmi/RIOT,mfrey/RIOT,BytesGalore/RIOT,toonst/RIOT,mtausig/RIOT,kaspar030/RIOT,jasonatran/RIOT,basilfx/RIOT,BytesGalore/RIOT,OTAkeys/RIOT,lazytech-org/RIOT,yogo1212/RIOT,ant9000/RIOT,basilfx/RIOT,aeneby/RIOT,A-Paul/RIOT,A-Paul/RIOT,kaspar030/RIOT,kYc0o/RIOT,mtausig/RIOT,yogo1212/RIOT,yogo1212/RIOT,ant9000/RIOT,josephnoir/RIOT,jasonatran/RIOT,toonst/RIOT,rfuentess/RIOT,mtausig/RIOT,OTAkeys/RIOT,A-Paul/RIOT,kYc0o/RIOT,smlng/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,toonst/RIOT,smlng/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT