mx23: config: Enable building of u-boot.sb binary

For i.MX23 we need to pass imx23 as elftosb param.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
diff --git a/Makefile b/Makefile
index 3305e8c..655ec39 100644
--- a/Makefile
+++ b/Makefile
@@ -508,6 +508,7 @@
 			$(obj)u-boot.ais
 
 # Specify the target for use in elftosb call
+ELFTOSB_TARGET-$(CONFIG_MX23) = imx23
 ELFTOSB_TARGET-$(CONFIG_MX28) = imx28
 
 $(obj)u-boot.sb:       $(obj)u-boot.bin $(obj)spl/u-boot-spl.bin