lualoader: don't botch disabling of color

When colors are disabled, color.escape{fg,bg} would return the passed in
color rather than the proper ANSI sequence for the color.
color.escape{fg,bg} would be wrong.

Instead return '', as the associated reset* functions will also return ''.
This should get rid of the funky '2' and '4' in the kernel selector if
you're booting serial.

Reported by: npn