[OE-core] LAYERSERIES

[Richard Purdie]

I finally found 5 mins to sit and look at where I'd gotten to with the
LAYERSERIES variable handling. I sent out discussion about this a while
ago but as a reminder:

We have a problem where people expect master branch of meta-X to work
with OE-Core master. If meta-X hasn't been maintained for a long time,
this may give all kinds of weird errors. This creates a large support
burden and means we struggle to spot unmaintained/un-updated layers.
We'd like to fix this.

In answer to this I added LAYERSERIES_CORENAMES to OE-Core and strongly
suggest layers then set:

LAYERSERIES_COMPAT_xxx = "sumo"

in their layer.conf where xxx is the collection name (used elsewhere in
layer.conf) to indicate which versions of OE-Core they're expected to
work against.

The code already exists to validate this with a fatal error for
mismatches. What was missing was:

a) to print a warning if LAYERSERIES_COMPAT_xxx is unset. I've a patch 
   queued for bitbake to add this.
b) add a check to yocto-check-layer and require this to be set for YP 
   Compatible v2. I've added
   https://bugzilla.yoctoproject.org/show_bug.cgi?id=12661
   to track this.

Yes, its late in the release and the warnings will be annoying but I
think its worth fixing this now so I'm pushing ahead to resolve this.

Cheers,

Richard