boards/common/cc26xx_cc13xx: improve default PROGRAMMER setting
boards/common/cc26xx_cc13xx: improve default PROGRAMMER setting

Only default to uniflash as default programmer if UNIFLASH_PATH is
set. If this isn't the case, `make flash` fails complaining about
missing `UNIFLASH_PATH` anyway.

miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kYc0o/RIOT,kYc0o/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT