Deprecate MAKE and/or CLEAN, introduce BUILD_FLAGS

[evelikov]

Looking across various projects using dkms (bbswitch, nvidia driver, openzfs and a bunch of others). The vast majority of them copy/paste the default (in-dkms) make/clean commands:

make -C $kernel_source_dir M=$dkms_tree/$module/$module_version/build
make -C $kernel_source_dir M=$dkms_tree/$module/$module_version/build clean"

That has a few interesting side effects - both kernel_source_dir (aka the linux path) and $dkms_tree are hard-coded in their config.

Glancing at the more complex modules (nvidia, openzfs) - they are glorified wrappers allowing behaviour changes (via funky indirection).

Off the top of my head, to handle these we could introduce a variable which gets added to the make command above. Say BUILD_FLAGS or BUILD_DEFINES (must include only -D...) or alike. As always the dkms.conf file will have the default, which can be altered by users with the existing conf.d override mechanism.

@scaronni @xuzhen any preferences?

@hadogenes considering your experience with openzfs what your take?