nut: Build serial drivers by default

Serial drivers don't add much to main NUT and appear to be common,
so make it possible for users to use NUT with serial UPS drivers without
a custom build.

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>

net/nut/Config.in

@@ -29,4 +29,4 @@
 		bool "Build with suport for serial drivers"
 		help
 			If you have a UPS connected via serial cable, select this.
-		default n
+		default y