Make some u-boot variables configurable

Author: gsingh93

Makefile

@@ -350,17 +350,17 @@ rootfs_clean:
 
 # Note: We're reusing the TARGET variable from the Linux build section
 
-UBOOT_SRC := $(ROOT_DIR)/u-boot
-UBOOT_OUT := $(OUT_DIR)/uboot/$(ARCH)
+UBOOT_SRC ?= $(ROOT_DIR)/u-boot
+UBOOT_OUT ?= $(OUT_DIR)/uboot/$(ARCH)
 UBOOT_CONFIG := $(UBOOT_OUT)/.config
 UBOOT_BIN := $(UBOOT_OUT)/u-boot.bin
 
 ifeq ($(ARCH),x86_64)
-  UBOOT_DEFCONFIG=qemu-x86_64_defconfig
+  UBOOT_DEFCONFIG ?= qemu-x86_64_defconfig
 else ifeq ($(ARCH),i386)
-  UBOOT_DEFCONFIG=qemu-x86_defconfig
+  UBOOT_DEFCONFIG ?= qemu-x86_defconfig
 else ifeq ($(ARCH),arm64)
-  UBOOT_DEFCONFIG=qemu_arm64_defconfig
+  UBOOT_DEFCONFIG ?= qemu_arm64_defconfig
 endif
 
 UBOOT_MAKE := \