cmake: esp32: Fix check for environment variable ESP_IDF_PATH
cmake: esp32: Fix check for environment variable ESP_IDF_PATH

Environment variables should be checked with if(DEFINED
ENV{ESP_IDF_PATH}).

if($ENV{ESP_IDF_PATH}) was not evaluating to true as it should have
when env[ESP_IDF_PATH] was set.

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

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