lualoader: fix setting of loader_color=NO in loader.conf(5)

Previously color.disabled would be calculated at color module load time,
then never touched again. We can detect serial boots beyond just what we're
told by loader.conf(5) so this works out in many cases, but we must
re-evaluate the situation after the config is loaded to make sure we're not
supposed to be forcing it enabled/disabled.

Discovered while trying to test r353872.