scripts:firmware: Fix check for U-Boot file types

On i.MX6 devices, the U-Boot file types are not set at all. Extent check for
the existance of the uboot_file_types var.

scripts/generate_firmware_package.py

@@ -147,7 +147,7 @@ def generate_firmware_package(
                 FirmwarePackageKeys.FNGSYS_UBOOT_CHECKSUM,
             ]
 
-        if uboot_file_types:
+        if uboot_file_types in locals():
             sbom_uboot_files = [
                 generate_subpackage(
                     files,