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

Add couple preparation steps: clean and configuration. Also add newly
added sysroot support info to cross-compile section.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191011002808.28206-16-ivan.khoronzhuk@linaro.org