Add environment variable for ARCH_VARIANT
Rename ARCH_VARIANT to ARCHVARIANT to mirror SUBARCH naming style
Change default variant to hardened+nomultilib (pain to type, easier to backspace or CTRL+U!)
Revert "Change default variant to hardened+nomultilib (pain to type, easier to backspace or CTRL+U!)"

This reverts commit 79b647e6dfe3dac4e4b44eb67a165a5578efdb49.
Improve interface name documentation.