[FS#924] uml: fail to build with "Missing site config for architecture"

[LEDE Bugs]

The following task has a new comment added:

FS#924 - uml: fail to build with "Missing site config for architecture"
User who did this - Florian Fainelli (ffainelli)

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UML is really only supported on i386 and x86_64, which is why the site logic is failing here because we don't have a sparc64 set of site files.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=924#comment3063