bpftool: Fix version string in recursive builds

bpftool: Fix version string in recursive builds

Forwarded: https://lore.kernel.org/bpf/20200813235837.GA497088@decadent.org.uk/T/#u

When bpftool is built as part of a Debian package build, which itself
uses make, "bpftool version" shows:

    bpftool vmake[4]: Entering directory /build/linux-5.8/tools/bpf/bpftool 5.8.8.0 make[4]: Leaving directory /build/linux-5.8

Although we pass the "--no-print-directory" option, this is overridden
by the environment variable "MAKEFLAGS=w".  Clear MAKEFLAGS for the
"make kernelversion" command.

I have no explanation for the doubled ".8" in the version string, but
this seems to fix that as well.

Signed-off-by: Ben Hutchings <benh@debian.org>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name bpftool-fix-version-string-in-recursive-builds.patch