bricks/ev3: download u-boot.bin

Download u-boot.bin instead of committing it to the repository. We
should not be committing binary blobs to source control. instead,
we can commit the SHA256 checksum to ensure we are getting what we
expect and only download the file when needed.

Author: dlech

bricks/_common/arm_none_eabi.mk

@@ -628,11 +628,19 @@ $(BUILD)/firmware-obj.bin: $(BUILD)/firmware.elf
 	$(ECHO) "`wc -c < $@` bytes"
 
 ifeq ($(PB_MCU_FAMILY),TIAM1808)
-UBOOT_FILE = $(PBTOP)/bricks/ev3/u-boot.bin
+
+# REVISIT: downloading things doesn't belong in a Makefile.
+$(BUILD)/u-boot.bin:
+	$(ECHO) "Downloading u-boot.bin"
+	$(Q)wget -q -O $@ https://github.com/pybricks/u-boot/releases/download/pybricks/v1.0.2/u-boot.bin
+	$(Q)echo "62fe9df8138a4676d61b72c6844f9e7c3cbfd85470b9cea1418abec4f79228ac  $(BUILD)/u-boot.bin" | sha256sum -c $< --strict
+
 MAKE_BOOTABLE_IMAGE = $(PBTOP)/bricks/ev3/make_bootable_image.py
+
 # For EV3, merge firmware blob with u-boot to create a bootable image.
-$(BUILD)/firmware-base.bin: $(BUILD)/uImage
-	$(Q)$(MAKE_BOOTABLE_IMAGE) $(UBOOT_FILE) $(BUILD)/uImage $(BUILD)/firmware-base.bin
+$(BUILD)/firmware-base.bin: $(MAKE_BOOTABLE_IMAGE) $(BUILD)/u-boot.bin $(BUILD)/uImage
+	$(Q)$^ $@
+
 else
 # For embeded systems, the firmware is just the base file.
 $(BUILD)/firmware-base.bin: $(BUILD)/firmware-obj.bin