download/u-boot: Try to update submodules as in coreboot script

The coreboot download script tries to update submodules, since coreboot
does use git submodules to retrieve and compile the projects it depends
on. Although U-Boot doesn't use submodules, try to update them anyway to
match the coreboot download script.

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

resources/scripts/download/u-boot

@@ -162,6 +162,13 @@ downloadfor() {
 		"download/u-boot" "${ubrevision}" "${board}" "${ubtree}"
 		return 1
 	fi
+
+	git -C "${ubtree}" submodule update --init || touch build_error
+	if [ -f build_error ]; then
+		printf "ERROR: %s: Unable to update submodules for tree '%s'\n" \
+			"${ubtree}"
+		return 1
+	fi
 }
 
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