[OE-core] [PATCH 2/2] Add api-doc class

[Zongchun YU]

>>It's not opt-in: once a recipe is using doxygen.bbclass then it has a build dependency on doxygen-native and *will* generate documentation.
>>This is needless overhead if you don't intend to read the documentation.
>
> I think if you intend to read the documentation. Just inherit this class. If not. there is no need to inherit this class.

So you're suggesting that if a user wants documentation generated they should know what recipes they can add "inherit api-doc" to?

I'll repeat my proposal: every recipe that can generate doxygen documentation inherits a doxygen class, and that class can look at a distro-level variable along the likes of GENERATE_API_DOCS (which a hypothetical non-stub gtk-doc class can also respect).
[Zongchun] OK. Can you explain the details to use GENERATE_API_DOCS? I am not sure the meaning. Thanks.