Newer kernels require "bison" and "flex", so to easy porting we should
add them to generated APKBUILDs by default. While we are it, clean up
the default set of dependencies a bit:
- installkernel is only needed when using "make zinstall", which
we are not using in downstream_package (it requires special kernel
configuration, so it's not really suitable for downstream...)
- most kernels seem to compile fine without gmp-dev and elfutils-dev,
so they should only be added when needed
- the kernel has its own headers, why do we need linux-headers?
(probably copied from Alpine, but anyway...)
- add openssl-dev since it is needed for building/signing kernel modules
Useful reference: https://www.kernel.org/doc/html/latest/process/changes.html
Although it mentions much more than what is really needed...
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