bpftools: add utility and library packages supporting eBPF usage

bpftools: add utility and library packages supporting eBPF usage

Add support for building bpftool and libbpf from the latest 5.8.3 kernel
sources, ensuring up-to-date functionality and fixes. Both are written to
be backwards compatible, which simplfies build and usage across different
OpenWRT image kernels.

'bpftool' is the primary userspace tool widely used for introspection and
manipulation of eBPF programs and maps. Two variants are built: a 'full'
version which supports object disassembly and depends on libbfd/libopcodes
(total ~500KB); and a 'minimal' version without disassembly functions and
dependencies. The default 'minimal' variant is otherwise fully functional,
and both are compiled using LTO for further (~30KB) size reductions.

'libbpf' provides shared/static libraries and dev files needed for building
userspace programs that perform eBPF interaction.

Several cross-compilation and build-failure problems are addressed by new
patches and ones backported from farther upstream:

  * 001-libbpf-ensure-no-local-symbols-counted-in-ABI-check.patch
  * 002-libbpf-fix-build-failure-from-uninitialized-variable.patch
  * 003-bpftool-allow-passing-BPFTOOL_VERSION-to-make.patch
  * 004-v5.9-bpftool-use-only-ftw-for-file-tree-parsing.patch

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>