[XS] Change in coreboot[main]: Fix no-op for empty arg in Makefile conditional

[Martin L Roth (Code Review)]

Attention is currently required from: Ashish Kumar Mishra, Balaji Manigandan, Nico Huber, Saurabh Mishra.

Martin L Roth has posted comments on this change by Balaji Manigandan. ( https://review.coreboot.org/c/coreboot/+/83887?usp=email )

Change subject: Fix no-op for empty arg in Makefile conditional
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Patch Set 6:

(1 comment)

Patchset:

PS4:
> ||Although I hit this issue due to different change, it can be replicated like: […]

Let's keep this comment unresolved.

Instead of adding the colon, it might be better to make sure that the third argument has a value and thrown an error if it doesn't.
```
ifeq ($(3),)
$(error Third value is empty in cbfs-add-cmd: $(1) $(2))
endif
```