Speed up unittests by not limitting Make to spawn more processes

[SeppoTakalo]

Description

Summary of change

Speed up unittests by not limitting Make to spawn more processes.

By default, the Make will use as many processes as specified by -j[N] parameter. But if there is a parameter -l[N] parameter given, it does not spawn any more processes, if load average is over N.

This greatly limits the effect of parallel build. On Mac OS X, I can see load average as high as 40, when building unittests. This is normal, and the system is still responsive.

Documentation

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@JuhPuur

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Release Notes

Summary of changes

Impact of changes

Migration actions required

[mbed-ci]

Test run: SUCCESS

Summary: 1 of 1 test jobs passed
Build number : 1
Build artifacts

[ciarmcom]

@SeppoTakalo, thank you for your changes.
@JuhPuur @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

@JuhPuur @ARMmbed/mbed-os-test Please review

[kimlep01]

This is a welcome speed boost

[0xc0170]

CI restarted

[mbed-ci]

Test run: SUCCESS

Summary: 1 of 1 test jobs passed
Build number : 2
Build artifacts