Adding in support for custom configurations

[e-liner]

This change will add in the custom _defconfig's to the buildroot
makefile so that when a _defconfig is in the custom directory it is
recognized correctly, and is able to be used.

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This change is

[williamspatrick]

@e-liner - This looks good to me and a (hopefully) useful contribution to the upstream buildroot. The master branch in this repository tracks upstream. I think we need this pull request to be against 2016.02-op-build instead.

We have tried to keep the op-build pretty close to master and then contribute the changes to the upstream buildroot. 2016.02-op-build is the most recent branch @shenki is maintaining for op-build.

@shenki - Can you review and give @e-liner advice on how to handle this?

[shenki]

Hey @e-liner, thanks for the patch - I think this is a great idea.

I tend to merge patches in myself rather than using the github PR button, don't worry about re-doing the pull request against our -op-build branch.

If you have time, please submit it to upstream:

https://buildroot.org/downloads/manual/manual.html#submitting-patches

If you're busy then I can submit it on your behalf.

The buildroot maintainers very responsive, so we will wait for their review, and then I will pick up the patch from upstream.

[williamspatrick]

@shenki - Submitted to upstream here: http://lists.busybox.net/pipermail/buildroot/2016-April/158666.html . I'll let you know if I see it merged in the upstream master.

[shenki]

It looks like upstream rejected this?

[e-liner]

Yes, upstream decided that this was too specific because it included the custom directory in the Makefile, they decided it would be better to add in a wildcard and then on our op-build side we can have a symlink from openpower/configs/custom to openpower/custom/configs.