Fix define for MPC5121 ADS board.

Originally the Makefile defines CONFIG_ADS5121_REV2 when configuring
for board mpc5121ads_rev2. The config in include/configs/mpc5121ads.h
expects a define CONFIG_MPC5121ADS_REV2 for this.
Therefore there are two defines used but only 1 is defined.
This patch renames references to CONFIG_ADS5121_REV2 to CONFIG_MPC5121ADS_REV2
to have one common define.

This patch fixes faulty mem size detection caused by wrong config in
include/configs/mpc5121ads.h

Signed-off-by: Mark Vels <mark.vels@team-embedded.nl>
diff --git a/Makefile b/Makefile
index 8df60fa..de4ceb9 100644
--- a/Makefile
+++ b/Makefile
@@ -700,7 +700,7 @@
 	: unconfig
 	@mkdir -p $(obj)include
 	@if [ "$(findstring rev2,$@)" ] ; then \
-		echo "#define CONFIG_ADS5121_REV2 1" > $(obj)include/config.h; \
+		echo "#define CONFIG_MPC5121ADS_REV2 1" > $(obj)include/config.h; \
 	fi
 	@$(MKCONFIG) -n $@ -a mpc5121ads powerpc mpc512x mpc5121ads freescale