uboot-mvebu: add missing UBOOT_MAKE_FLAGS variable

This patch removes "/bin/sh: HOSTCPPFLAGS: command not found" errors douring build.

Signed-off-by: Marko Ratkaj <marko.ratkaj@sartura.hr>

package/boot/uboot-mvebu/Makefile

@@ -16,6 +16,8 @@ PKG_HASH:=f54baf3f9325bf444c7905f3a5b6f83680edb1e6e1a4d5f8a5ad80abe885113f
 include $(INCLUDE_DIR)/u-boot.mk
 include $(INCLUDE_DIR)/package.mk
 
+UBOOT_MAKE_FLAGS:=
+
 define U-Boot/Default
   BUILD_TARGET:=mvebu
   HIDDEN:=1