Fix: Use YAML config by default

Sice the JSON config was added and default, that breaks the
original behavior of taking it from the YAML config, it should
be used by default.

Tested: When `--enable-use_json` was added in phosphor-led-manager_git.bbapped,
the JSON config was used, otherwise, the YAML config was used.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I599db99c132fbf402a262e0ee04b2ece5efb32da
diff --git a/Makefile.am b/Makefile.am
index 2660b32..7f58aa3 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -6,7 +6,7 @@
                 manager.cpp \
                 group.cpp
 
-if !LED_USE_JSON
+if !WANTS_JSON
 BUILT_SOURCES = led-gen.hpp
 CLEANFILES = led-gen.hpp