Collect required kernel features in metapackage

[wkz]

We should investigate if we can create a Buildroot package whose only purpose is to enable generic kernel features needed by Infix.

That would allow us to trim down the per-board configs to only define required hardware support.

It would complicate make linux-update-defconfig it bit though, since you would get the union of the base config plus all options from the meta package. We might want to automate the disentanglement of that somehow.