Debian Policy "4.9. Main building script: debian/rules" requires
"debian/rules must start with the line #!/usr/bin/make -f". [1]

Currently, Kbuild does not follow this policy.

When Kbuild generates debian/rules, "#!$(command -v $MAKE) -f" is
expanded by shell. The resuling string may not be "#!/usr/bin/make -f".

There was a reason to opt out the Debian policy.

If you run '/path/to/my/custom/make deb-pkg', debian/rules must also be
invoked by the same Make program. If #!/usr/bin/make were hard-coded in
debian/rules, the sub-make would be executed by a possibly different
Make version.

This is problematic due to the MAKEFLAGS incompatibility, especially the
job server flag. Old Make versions used --jobserver-fds to propagate job
server file descriptors, but Make >= 4.2 uses --jobserver-auth. The flag
disagreement between the parent/child Makes would result in a process
fork explosion.

However, having a non-standard path in the shebang causes another issue;
the generated source package is not portable as such a path does not
exist in other build environments.

This commit solves those conflicting demands.

Hard-code '#!/usr/bin/make -f' in debian/rules to create a portable and
Debian-compliant source package.

Pass '--rules-file=$(MAKE) -f debian/rules' when dpkg-buildpackage is
invoked from Makefile so that debian/rules is executed by the same Make
program as used to start Kbuild.

[1] https://www.debian.org/doc/debian-policy/ch-source.html#main-building-script-debian-rulesSigned-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>