payloads/U-Boot: Do a shallow clone

We don't need all the git history. Speed up the build by doing a shallow
clone.

Signed-off-by: Simon Glass <sjg@chromium.org>
Change-Id: If31c9dd158aa23f242e4fd145449ef7502fb1ab1
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59602
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/payloads/external/U-Boot/Makefile b/payloads/external/U-Boot/Makefile
index 1d1d332..90777a5 100644
--- a/payloads/external/U-Boot/Makefile
+++ b/payloads/external/U-Boot/Makefile
@@ -26,7 +26,7 @@
 
 $(project_dir):
 	echo "    Cloning $(project_name) from Git"
-	git clone $(project_git_repo) $(project_dir)
+	git clone --depth 1 $(project_git_repo) $(project_dir)
 
 fetch: $(project_dir)
 ifeq ($(CONFIG_UBOOT_MASTER),y)