u-boot: Make u-boot.imx depend on docker-uboot-imx

If we don't do this, we end up in spirals of death relating to pbuilder
attempting to run in multiple instances on a machine.

Change-Id: I63b1449a357c3274bb8497613de758e4bb900ff5
diff --git a/u-boot.mk b/u-boot.mk
index 895dccb..00cad1a 100644
--- a/u-boot.mk
+++ b/u-boot.mk
@@ -20,7 +20,7 @@
 
 u-boot: $(PRODUCT_OUT)/u-boot.imx
 
-$(PRODUCT_OUT)/u-boot.imx: uboot-imx | out-dirs
+$(PRODUCT_OUT)/u-boot.imx: docker-uboot-imx | out-dirs
 	dpkg --fsys-tarfile $(PRODUCT_OUT)/packages/uboot-imx*.deb | \
 	tar --strip-components 2 -C $(PRODUCT_OUT) -xf - ./boot/u-boot.imx