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

stand/lua/color.lua

@@ -58,7 +58,7 @@ color.disabled = not color.isEnabled()
 
 function color.escapefg(color_value)
 	if color.disabled then
-		return color_value
+		return ''
 	end
 	return core.KEYSTR_CSI .. "3" .. color_value .. "m"
 end
@@ -72,7 +72,7 @@ end
 
 function color.escapebg(color_value)
 	if color.disabled then
-		return color_value
+		return ''
 	end
 	return core.KEYSTR_CSI .. "4" .. color_value .. "m"
 end

stand/lua/screen.lua

@@ -47,14 +47,14 @@ end
 
 function screen.setforeground(color_value)
 	if color.disabled then
-		return color_value
+		return
 	end
 	printc(color.escapefg(color_value))
 end
 
 function screen.setbackground(color_value)
 	if color.disabled then
-		return color_value
+		return
 	end
 	printc(color.escapebg(color_value))
 end