tegra-uefi-capsules: add missing DTB_EXTRA_DEPS dependency

[kekiefer]

The collection of dtb overlays used during tegraflash package creation can come from sources other than l4t or the kernel. Most commonly this is the virtual/dtb package, but it can be overridden, and is specified by the DTB_EXTRA_DEPS variable.

The migration from creating bup payloads to creating only capsules inadvertantly dropped this dependency, so this change adds it back.

[madisongh]

Thanks

[kekiefer]

Thanks, Matt. Please cherry-pick for scarthgap as well.

[madisongh]

Yep, will do that shortly.