kernel: alternative configuration

[tzafrir-mellanox]

Kernel is configured by querying the kernel build system directly, and
kernel directory can be built independetly of autoconf.

This solves several separate issues:

• Having to carry all of autoconf to the DKMS directory
• The compiler that autoconf detects by default may not be the one
  that the kernel needs.
  • It is even OK to not have "gcc" and have "gcc-14" if that is what
    the kernel needs, but autoconf will still be picky.
• Various other issues are best detected by the kernel build system
  directly, and not trying ot second-guess it, as this occasionally
  changes.
  • One exception: clang support requires explicitly setting
    LLVM=1 . Not done automatically for now.

Changes:

• m4/ac_kernel_checks.m4 has mostly been replaced by the script
  kernel/config_kernel
  • This script can also be run manually. See dkms.conf .
  • The script does not include tests. They are in files under
    kernel/test_cases/
  • kernel/ can be run without autoconf: kernel/Kbuild was simplified.
  • And alternative Makefile for it is provided as kernel/Makefile.dkms
• Using relative path to generated include file rather than path
  in a generated file from autoconf

The test works by creating a kernel module for each test and
building all of them with the kernel build system and with the -k
option of make and check which of them built successfully.

As an initial stage, we run a test build of a single empty module
just to be sure that the build system works.

To simplify debugging, kernel tests can be re-run separately (after
main script was run) using e.g.:

kernel/config_kernel HAVE_PUD_LEAF_MACRO

[tzafrir-mellanox]

Some notes:

• On my system, configure script runs slightly faster: tests are slower to run than original configure (though more reliable), but are run in parallel (by the kernel build system).
• This change may break out-of-tree builds. Do you have any existing users of such? If so, I can test and try to fix.
• I opted to include tests in multiple files. Alternatively, it is possible to embed them in a autoconf m4 file. This is how it's done in mlnx-ofa_kernel (compat/config/rdma.m4). But if you prefer it that way, it could be changed.
• As noted, mlnx-ofa_kernel already has various tests of kernel features in basically the same format that we can use, if needed.