u-boot.mk: add UBOOT_MAKE_FLAGS variable similar to MAKE_FLAGS

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/u-boot.mk

@@ -76,7 +76,8 @@ DTC=$(wildcard $(LINUX_DIR)/scripts/dtc/dtc)
 define Build/Compile/U-Boot
 	+$(MAKE) $(PKG_JOBS) -C $(PKG_BUILD_DIR) \
 		CROSS_COMPILE=$(TARGET_CROSS) \
-		$(if $(DTC),DTC="$(DTC)")
+		$(if $(DTC),DTC="$(DTC)") \
+		$(UBOOT_MAKE_FLAGS)
 endef
 
 define BuildPackage/U-Boot/Defaults