nvidia-kernel-oot: build in B with Devtool

[dchvs]

Enable external builds, such as those initiated by Devtools, to utilize the B directory for building instead of modifying the source code directory. This prevents the source code from being tainted by build artifacts, which currently results in issues like the following:

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        hwpm/drivers/tegra/hwpm/.Module.symvers.cmd
        hwpm/drivers/tegra/hwpm/.modules.order.cmd
        hwpm/drivers/tegra/hwpm/.nvhwpm.ko.cmd
        hwpm/drivers/tegra/hwpm/.nvhwpm.mod.cmd
        hwpm/drivers/tegra/hwpm/.nvhwpm.mod.o.cmd
        hwpm/drivers/tegra/hwpm/.nvhwpm.o.cmd
        hwpm/drivers/tegra/hwpm/Module.symvers
        hwpm/drivers/tegra/hwpm/common/.allowlist.o.cmd
        hwpm/drivers/tegra/hwpm/common/.aperture.o.cmd
        hwpm/drivers/tegra/hwpm/common/.init.o.cmd
        hwpm/drivers/tegra/hwpm/common/.ip.o.cmd
        hwpm/drivers/tegra/hwpm/common/.resource.o.cmd
        hwpm/drivers/tegra/hwpm/common/allowlist.o
        hwpm/drivers/tegra/hwpm/common/aperture.o

[madisongh]

Does this actually produce the correct results? There are many makefiles and scripts in that tree, and when I put together the recipe I wasn't 100% sure that they would work properly with B != S. The build might not fail, but you could end up with non-functioning drivers. It would have been hard to verify correctness so I didn't think it was worth the risk. One can always use -f on the devtool finish command to work around the in-tree generated files.

[dchvs]

Let me know if you'd like me to proceed with some verifications, or if you'd prefer not to merge this change for now —in that case feel free close the PR.