[Skiboot] skiboot-5.0-rc3

[Stewart Smith]

Hi all,

I've tagged skiboot-5.0-rc3 today. The only outstanding issue I'm aware
of is in regards to progress sensor on astbmc issues.

Changes since rc2:
- add support for cec_power_down on mambo
- external/opal-prd: Use link register for cross-endian branch
- opal header file rework, Linux and skiboot now very closely match (API
  in opal-api.h)
- libflash: don't use the low level interface if it doesn't exist
- libflash/file: add file abstraction for libflash
- external: create a GUARD partition parsing utility

Now is the time to hammer things and work out if there's anything else
we should fix.