Makefile.include: flash: do not peek into MAKECMDGOALS.
Makefile.include: flash: do not peek into MAKECMDGOALS.

When flash-only was introduced (in #8373), the `flash` rule was
made conditionally dependent on `all` by looking for `flash-only`
in MAKECMDGOALS. This was done to avoid code duplication.

There's a cleaner way, by using canned recipes. When we upgrade the
requirements to gnu make 4, the flash recipe can be defined as ?=.

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