cmake: qemu: Restore QEMU_INSTANCE option handling.
cmake: qemu: Restore QEMU_INSTANCE option handling.

QEMU_INSTANCE should be a parameter to *make*, not cmake, so people
can run few instances of the same sample without building each of
them separately in separate dirs.

Signed-off-by: Paul Sokolovsky <35cb557f011771c94f127d6fef99f804b62203f5@linaro.org>

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