powerpc: T4160RDB: Separate from T4240RDB in Kconfig

Use TARGET_T4160RDB to simplify Kconfig options.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/board/freescale/t4rdb/Makefile b/board/freescale/t4rdb/Makefile
index 4f29eea..c15560e 100644
--- a/board/freescale/t4rdb/Makefile
+++ b/board/freescale/t4rdb/Makefile
@@ -7,6 +7,7 @@
 ifdef CONFIG_SPL_BUILD
 obj-y	+= spl.o
 else
+obj-$(CONFIG_TARGET_T4160RDB)	+= t4240rdb.o
 obj-$(CONFIG_T4240RDB)	+= t4240rdb.o
 obj-y			+= cpld.o
 obj-y			+= eth.o