tools/bpftool: Add bpftool support for split BTF

Add ability to work with split BTF by providing extra -B flag, which allows to
specify the path to the base BTF file.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20201105043402.2530976-12-andrii@kernel.org