by having two separate derivations for patching the kernel source tree
and building it, we have to copy said source trees from one store
location to another which takes non-neglible time on spinning rust
(literally minutes on my machine). Replace with a single derivation
that can do more things on one tree in-place
Some kernel config symbols we need to enable in some configurations
don't have a "prompt" entry in the Kconfig files, meaning that
make oldconfig (or any other make fooconfig) won't change them.
So we add a Kconfig.local where they can be selected
In this patch we hardcode the required symbol. Next step would be
to generalise so that they could be specified in something like a
config.kernel.hiddenConfig attrset
Sometimes config options get silenty removed because they have
dependencies on other options that haven't been enabled. If you add
those as attributes here, the kernel build will check they're _still_
present in the config after make oldconfig has run.
