skiboot 5.1.5 for op-build v1.7

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We're going to do a skiboot 5.1.5 with currently two fixes scheduled:

• https://patchwork.ozlabs.org/patch/521595/ - PHB3: Fix wrong PE number in error injection
• fix indirect scoms for centaur

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this pull request will fix this issue: https://github.com/open-power/op-build/pull/285