doc/overview: flesh out overview
Skiboot overview
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-Skiboot is firmware, loaded by the FSP. Along with loading the bootloader,
-it provides some runtime services to the OS (typically Linux).
+Skiboot is boot and runtime firmware for OpenPOWER systems.
+It's loaded by earlier boot firmware (typically Hostboot).
+Along with loading the bootloader, it provides some runtime
+services to the OS (typically Linux).
Source layout
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