[RFC PATCH v2] Allow remote sstate cache fetches when BB_NO_NETWORK set

[Joe MacDonald <joe@deserted.net>]

Originally I had two changes, on to oe-core's sstate.bbclass and one in
bitbake's fetcher.  Following on from the feedback on the bitbake-devel list,
here's version two, isolated to sstate.bbclass.

My objective here is to support a very specific scenario.  I have a well-fed
sstate cache on my internal network.  I want to use it for accelerating builds
across a variety of machines where NFS is not really an option.  I don't want
bitbake to ever attempt to fetch sources for packages, though, so I'm always
using BB_NO_NETWORK.

The proposal I have is adding a new local.conf variable that will allow cache
fetches only to be exempt from BB_NO_NETWORK restrictions.  If it is set and
BB_NO_NETWORK is not set, there's no change in behaviour.  If it is not set and
BB_NO_NETWORK is set, again, no change in today's behaviour.  If both are set,
however, for sstate mirror fetch attempts we will temporarily mask off
BB_NO_NETWORK, allowing the fetch to complete for the sstate object, but still
disable network access for remote sources.

-- 
-Joe MacDonald.
:wq