makechrootpkg: Fix anti-pattern when checking for enabled features

Don't use error-prone logic e.g.
foo=true; if $foo ...

This completely fails to act as expected when the variable is unset
because of unrelated bugs.

While this merely causes the default behavior to be "false" rather than
"true" in such cases, it is better to fail to enable explicitly
requested behavior (which will be noticed by the user) than to simply
upgrade to this behavior for free (which may not seem to have any
obvious cause).

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>