scripts: runner: nrfjprog: remove BOARD environment requirement
scripts: runner: nrfjprog: remove BOARD environment requirement

The BOARD variable has been removed from the environment provided to
runners. It's not being used to flash the board, so just remove the
check for it to avoid an exception at runtime.

Signed-off-by: Marti Bolivar <fd5da480b758a78208111e6ae5c6806e1f887137@opensourcefoundries.com>

Vudentz/zephyr,kraj/zephyr,zephyriot/zephyr,punitvara/zephyr,kraj/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,galak/zephyr,kraj/zephyr,explora26/zephyr,kraj/zephyr,ldts/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,aceofall/zephyr-iotos,Vudentz/zephyr,aceofall/zephyr-iotos,aceofall/zephyr-iotos,punitvara/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,nashif/zephyr,nashif/zephyr,aceofall/zephyr-iotos,galak/zephyr,galak/zephyr,nashif/zephyr,explora26/zephyr,ldts/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,mbolivar/zephyr,ldts/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,punitvara/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,zephyriot/zephyr,nashif/zephyr,zephyriot/zephyr