tools/bpftool: Support passing BPFTOOL_VERSION to make

This change facilitates out-of-tree builds, packaging, and versioning for
test and debug purposes. Defining BPFTOOL_VERSION allows self-contained
builds within the tools tree, since it avoids use of the 'kernelversion'
target in the top-level makefile, which would otherwise pull in several
other includes from outside the tools tree.

Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200917115833.1235518-1-Tony.Ambardar@gmail.com