powerpc: T4240RDB: Remove macro CONFIG_T4240RDB

Use CONFIG_TARGET_T4240RDB instead.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/board/freescale/t4rdb/Makefile b/board/freescale/t4rdb/Makefile
index c15560e..209983a 100644
--- a/board/freescale/t4rdb/Makefile
+++ b/board/freescale/t4rdb/Makefile
@@ -8,7 +8,7 @@
 obj-y	+= spl.o
 else
 obj-$(CONFIG_TARGET_T4160RDB)	+= t4240rdb.o
-obj-$(CONFIG_T4240RDB)	+= t4240rdb.o
+obj-$(CONFIG_TARGET_T4240RDB)	+= t4240rdb.o
 obj-y			+= cpld.o
 obj-y			+= eth.o
 obj-$(CONFIG_PCI)	+= pci.o