Merge branch 'skel-fixes'

Andrii Nakryiko says:

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Simplify skeleton usage by embedding source BPF object file inside skeleton
itself. This allows to keep skeleton and object file in sync at all times with
no chance of confusion.

Also, add bpftool-gen.rst manpage, explaining concepts and ideas behind
skeleton. In examples section it also includes a complete small BPF
application utilizing skeleton, as a demonstration of API.

Patch #2 also removes BPF_EMBED_OBJ, as there is currently no use of it.

v2->v3:
- (void) in no-args function (Alexei);
- bpftool-gen.rst code block formatting fix (Alexei);
- simplified xxx__create_skeleton to fill in obj and return error code;

v1->v2:
- remove whitespace from empty lines in code blocks (Yonghong).
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Signed-off-by: Alexei Starovoitov <ast@kernel.org>