[Exporters] Fix generic ARM CPU target in uvision

[sarahmarshy]

When a target does not have a device name, we want to use a generic ARM cpu export target. This fixes a bug in the implementation of that functionality.

Reproduce:
Run mbed export on a target without a device name. It should not fail, as the intended logic was for this line to find a generic CPU target. However, as it is, it will fail.

@theotherjimmy

[sarahmarshy]

/morph export-build

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph export-build

Output

mbed Build Number: 37

All exports and builds passed!

[bridadan]

@sarahmarshy If it wasn't accessing the cache's index, which index was it accessing? Could you explain the different indexes a bit?

[sarahmarshy]

It wasn't accessing anything. That variable doesn't exist. It threw an exception. Additionally, this is a static method, so really the only thing it can access are other static variables (cache being the only one).

[bridadan]

Ah I see, when did this exception occur? The exporter tests have been working for sometime now. Is this only when building up a new index?

[sarahmarshy]

When determining support. All the targets that threw this exception would be considered "skipped."

[bridadan]

Great, thanks for the clarification @sarahmarshy!