Fix more misuses of kernel_url instead of uboot_url

c2505d88 did only part of the job

build-image

@@ -211,7 +211,6 @@ echo > "${meta_yml}"
 
 if [ -n "${kernel_url}" ]; then
         fetch_kernel
-        fetch_uboot "${uboot_type}"
         for deb in files/linux-image-*-*_arm64.deb; do
                 ! [[ "${deb}" =~ dbg ]] || continue
                 if [ -n "${kernel_deb}" ]; then
@@ -224,6 +223,11 @@ if [ -n "${kernel_url}" ]; then
         custom_package_opt=(--custom-package "${kernel_deb}")
 fi
 
+if [ -n "${uboot_url}" ]; then
+        fetch_uboot "${uboot_type}"
+        echo "Using unpackaged uboot from ${uboot_url}"
+fi
+
 [ -z "${kernel_deb}" ] || echo "Using kernel ${kernel_deb}"
 
 # Preserve newlines as '|'
@@ -265,7 +269,7 @@ set +x
 
 if [ "${arch}" == "arm64" ]; then
     if [ ! -f "files/uboot-${uboot_type}/m4.bin" ]; then
-        wget -Pfiles/uboot-${uboot_type}/ "${kernel_url}/uboot-${uboot_type}/m4.bin"
+        wget -Pfiles/uboot-${uboot_type}/ "${uboot_url}/uboot-${uboot_type}/m4.bin"
     fi
 
     if [ "${want_boot_image}" == 1 ]; then