Fix broken boolean options
Fix broken boolean options

This commit allows calling
```
configure.py --opt1 --no-opt2
```
to enable opt1 and disable op2 from their defaults.

Previous boolean handling was utterly broken.

iree-org/iree-llvm-sandbox,iree-org/iree-llvm-sandbox,iree-org/iree-llvm-sandbox,iree-org/iree-llvm-sandbox