makefiles/tools/serial.inc.mk: unexport TERM* variables
makefiles/tools/serial.inc.mk: unexport TERM* variables

TERMPROG and TERMFLAGS variables do not need to be exported as they are
used directly by a make receipe.

Exporting them prevents overwriting 'RIOT_TERMINAL' in the test.

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