lib: descend into sub directories only when it is necessary

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/lib/lzma/Makefile b/lib/lzma/Makefile
index 25c6797..f8eda06 100644
--- a/lib/lzma/Makefile
+++ b/lib/lzma/Makefile
@@ -10,4 +10,4 @@
 
 CFLAGS += -D_LZMA_PROB32
 
-obj-$(CONFIG_LZMA) += LzmaDec.o LzmaTools.o
+obj-y += LzmaDec.o LzmaTools.o