[yocto] SRCREV for BBLAYERS

[Khem Raj]

> As pointed out above, angstrom has its own system. Other projects (e.g.
> gumstix[2] and fsl[3]) make use of "repo". Another option is "git
> submodules".
>
> Whether a mechanism like this would be, could be, or should be added to
> OE? That's a tough one. Personally I would lean towards "yes".

There already is a tool called combo-layer to create unified view by
combining layers
in OE-Core but its not one of above

 I think
> it would make OE much easier to use (especially for newcomers) if the
> combinations of layers and which commits to use from those layers were
> specified as part of a, say, DISTRO's metadata. On the other hand others
> would object simply because this is a not-uncommon git issue that is
> already solved by existing, external tools.