sudo nvram boot-args="iog=0x0"
This reverses the “Clamshell” mode for Apple’s laptop systems, where when you close the display but connect the system to an external monitor and keyboard the system will stay awake. After running this command, when connecting an external monitor, the internal display will be disabled, which can be beneficial in some situations such as those where you are mirroring your desktop but wish to run the external display at a higher resolution than your laptop can run.
To disable these features and have the system boot normally without any extra options, you can erase them from the nvram by either resetting it or, more specifically, by running either of the following commands in the Terminal (these will reset the boot arguments instead of resetting all the nvram variables):
sudo nvram boot-args="" sudo nvram -d boot-args