cmake: extensions: support list arguments to set_ifndef()
cmake: extensions: support list arguments to set_ifndef()

I keep tripping over not being able to use set_ifndef to set a value
to a list, like this:

set_ifndef(VARIABLE value1 value2 value3...)

Allow that to work without changing its existing behavior when called
with one argument.

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

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