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config.mk in board directory is obsolete and should be removed.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: NStefano Babic <sbabic@denx.de>

Makefile

@@ -352,7 +352,7 @@ $(obj)u-boot.img:	$(obj)u-boot.bin
 		-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