Add savedefconfig Makefile target

[InfiniteCoder01]

Very simple. Allows easily getting defconfigs (for make olddefconfig) for boards configured with make menuconfig (just by running make savedefconfig)

[JamesH1978]

Thankyou for submitting a PR, please can you sign off your as mentioned in point 3 in https://github.com/Klipper3d/klipper/blob/master/docs/CONTRIBUTING.md#what-to-expect-in-a-review

Thanks James

[InfiniteCoder01]

I've added a sign-off line to commit description as in https://github.com/Klipper3d/klipper/blob/master/docs/CONTRIBUTING.md#format-of-commit-messages

[github-actions[bot]]

Thank you for your contribution to Klipper. Unfortunately, a reviewer has not assigned themselves to this GitHub Pull Request. All Pull Requests are reviewed before merging, and a reviewer will need to volunteer. Further information is available at: https://www.klipper3d.org/CONTRIBUTING.html

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