Fix CROSS_COMPILE of U-boot

U-boot was given a fixed CROSS_COMPILE that does not match the one used
by GCC 9.2. Use the UBOOT_TARGET variable instead.

Test: make test
Change-Id: I62c1cc0bec65a7c380b2071beddfa88ef8b3238d
diff --git a/Makefile.root b/Makefile.root
index 56c19ea..72fba1b 100644
--- a/Makefile.root
+++ b/Makefile.root
@@ -184,7 +184,7 @@
 $(eval $(call define_toolchain,$(UBOOT_TOOLCHAIN),UBOOT_))
 UBOOT_MAKE := \
 	PATH=$(UBOOT_TOOLCHAIN_BIN):$(PATH) \
-	CROSS_COMPILE=aarch64-linux-gnu- \
+	CROSS_COMPILE=$(UBOOT_TARGET)- \
 	$(MAKE) \
 	V=$(UBOOT_VERBOSE) \
 	-C $(UBOOT_SRC) \