Fix convert-constants handling of booleans

It turns out that, in Python, booleans are also integers. So they fall
under the “isinstance(value, int)” case, resulting in all enable*
constants being integers in Haskell, which is not nice, even though
we're not using them directly today.

Patch simply adds a special casing for booleans, before integers.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>