Makefile: change rule to build IMX image

config.mk in board directory is obsolete and should be removed.
The patch allows to get rid of own config.mk adding the imximage.cfg
file to the options in the boards.cfg

Signed-off-by: Stefano Babic <sbabic@denx.de>
diff --git a/Makefile b/Makefile
index 713dba1..c463a8a 100644
--- a/Makefile
+++ b/Makefile
@@ -352,7 +352,7 @@
 		-d $< $@
 
 $(obj)u-boot.imx:       $(obj)u-boot.bin
-		$(obj)tools/mkimage -n $(IMX_CONFIG) -T imximage \
+		$(obj)tools/mkimage -n  $(CONFIG_IMX_CONFIG) -T imximage \
 		-e $(CONFIG_SYS_TEXT_BASE) -d $< $@
 
 $(obj)u-boot.kwb:       $(obj)u-boot.bin