Remove reference to non-existent file

Change-Id: I3555a348799e65d5af5193fdfe7c5476ab970feb
diff --git a/sdcard.mk b/sdcard.mk
index e8f32a3..4ffb84b 100644
--- a/sdcard.mk
+++ b/sdcard.mk
@@ -33,7 +33,6 @@
 $(PRODUCT_OUT)/sdcard.img: sdcard-allocate \
                            $(ROOTDIR)/build/rootfs.mk \
                            $(ROOTDIR)/build/boot.mk \
-                           $(ROOTDIR)/build/u-boot.mk \
                            $(ROOTDIR)/board/fstab.sdcard \
                            | $(PRODUCT_OUT)/u-boot.imx \
                            $(PRODUCT_OUT)/boot.img \