This short tutorial by user shows you how to set and unset loader variables during boot loader prompt for FreeBSD. Follow the link below for the full set of instructions.

freebsdImportant question is why you need to set and unset variables at boot time???

The answer is when something is stopping you from booting your system then this may help you make your day easier. There are so many variables set by default in the system by various kernel subsystems or driver modules. you can find the default variables in /boot/defaults/loader.conf and user set variables in /boot/loader.conf file. You can see all the variables set at the loader prompt by typing the command “show” (without the quotes).

So lets get started to know how you can manipulate loader variables. Remember that all the commands named below are valid for boot loader prompt in FreeBSD only and these commands will not work on a command shell. Press ESCape key on the keyboard when you are at FreeBSD boot prompt to enter into the loader prompt.

Full tutorial: http://www.explorelinux.com/freebsd-set-unset-loader-variables-boot-loader-prompt/