[Skiboot] [PATCH] README: Update build instructions

[Stewart Smith]

Oliver O'Halloran <oohall at gmail.com> writes:
> Add some more explicit instructions about how to install and use cross
> compilers.
>
> Signed-off-by: Oliver O'Halloran <oohall at gmail.com>
> ---
> Can someone verify the ubuntu instructions work on an x86 laptop.

Even better, I stole dependencies from opal-ci/Dockerfile-* and put them
in.

Merged to master as of 8fbf67e5b1bd5295e7795f7534832fb44889b9e6


-- 
Stewart Smith
OPAL Architect, IBM.