imx: colibri_imx6: fix splash logo drawing

Define "splashimage" variable in the default environment
to enable splash screen drawing.

Signed-off-by: Anatolij Gustschin <agust@denx.de>

include/configs/colibri_imx6.h

@@ -195,6 +195,7 @@
 		"load ${interface} ${drive}:1 ${loadaddr} flash_blk.img && " \
 		"source ${loadaddr}\0" \
 	"splashpos=m,m\0" \
+	"splashimage=" __stringify(CONFIG_LOADADDR) "\0" \
 	"vidargs=video=mxcfb0:dev=lcd,640x480M@60,if=RGB666 " \
 		"video=mxcfb1:off fbmem=8M\0 "