samples/bpf: Add preparation steps and sysroot info to readme

Note that LLVM's tool 'llc' must support target 'bpf', list version

and supported targets with command: ``llc --version``

Clean and configuration

-----------------------

It can be needed to clean tools, samples or kernel before trying new arch or

after some changes (on demand)::

 make -C tools clean

 make -C samples/bpf clean

 make clean

Configure kernel, defconfig for instance::

 make defconfig

Kernel headers

--------------

Cross compiling samples

-----------------------

In order to cross-compile, say for arm64 targets, export CROSS_COMPILE and ARCH

environment variables before calling make. This will direct make to build

samples for the cross target.

environment variables before calling make. But do this before clean,

cofiguration and header install steps described above. This will direct make to

build samples for the cross target::

 export ARCH=arm64

 export CROSS_COMPILE="aarch64-linux-gnu-"

make samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang

Headers can be also installed on RFS of target board if need to keep them in

sync (not necessarily and it creates a local "usr/include" directory also)::

 make INSTALL_HDR_PATH=~/some_sysroot/usr headers_install

Pointing LLC and CLANG is not necessarily if it's installed on HOST and have

in its targets appropriate arm64 arch (usually it has several arches).

Build samples::

 make samples/bpf/

Or build samples with SYSROOT if some header or library is absent in toolchain,

say libelf, providing address to file system containing headers and libs,

can be RFS of target board::

 make samples/bpf/ SYSROOT=~/some_sysroot