serial.inc.mk: disable local echo in picocom.
serial.inc.mk: disable local echo in picocom.

Due to a recent fix in shell.c, remote echo is now working as originally
intended. Local echo must be disabled or otherwise it will add up to the
remote one, causing a character-by-character double echoing.

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