download/u-boot: Use GitHub mirror as fallback

The coreboot download script uses GitHub as a fallback if the upstream
coreboot is unavailable, use a similar fallback for U-Boot as well.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>

resources/scripts/download/u-boot

@@ -124,6 +124,12 @@ downloadfor() {
 		git clone https://source.denx.de/u-boot/u-boot.git \
 		    "${uboot_dir}" || \
 		    rm -Rf "${uboot_dir}"
+		if [ ! -d "${uboot_dir}" ]; then
+			printf "WARNING: Upstream failed. Trying backup github repository:\n"
+			git clone https://github.com/u-boot/u-boot.git \
+				"${uboot_dir}" || \
+				rm -Rf coreboot
+		fi
 		if [ ! -d "${uboot_dir}" ]; then
 		    printf \
 			"ERROR: %s: Problem with git-clone. Network issue?\n" \