packaging: Add bpftool subpackage and build target

packaging: Add bpftool subpackage and build target

Update the linux-rpi4.spec file to build and package bpftool as a
separate subpackage, named linux-kernel-bpftool. By doing so, bpftool
(and its dependencies) can be managed more flexibly. The bpftool utility
is crucial for managing, loading, and debugging eBPF programs, making it
easier to inspect and troubleshoot eBPF-related features.

Changes:
 - Add a dedicated build target for bpftool under tools/bpf/bpftool.
 - Install bpftool and supporting files into /usr/sbin.
 - Create a new subpackage (linux-kernel-bpftool) that provides the
   bpftool binary.

The skeleton generation capability of bpftool is specifically required
for our resourced eBPF build, ensuring that the compiled BPF object can
be integrated seamlessly with user space. This will simplify building
and maintaining eBPF programs for low-memory monitoring or other
functionalities.

Add required manifest file for userspace utility bpftool.

Change-Id: Ie8f83923c537915d7f313fa84e49fe2eaa0877dc
Signed-off-by: Michal Wilczynski <m.wilczynski@samsung.com>