imx: add missing linux dependency for flash-image

Adds a missing linux dependency for the flash-image target.

This should fix erros like:
  CC [M]  drivers/gpu/drm/nouveau/nvkm/subdev/i2c/bus.o
mformat -i .../build/../out/boot.img.fat -n 64 -h 255 -T 131072 -v "BOOT IMG" -C ::
mcopy -i .../build/../out/boot.img.fat .../build/../linux/arch/arm64/boot/Image ::
.../build/../linux/arch/arm64/boot/Image: No such file or directory

Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/imx.mk b/imx.mk
index caa5073..a5e76fc 100644
--- a/imx.mk
+++ b/imx.mk
@@ -214,7 +214,7 @@
 endif
 
 .PHONY: flash-image
-flash-image: buildroot mkimage
+flash-image: buildroot mkimage linux
 	$(MAKE) flash-image-only
 
 .PHONY: flash-image-only