Add building of the bpf(2) syscall commands documentation as part of the
docs building step in the build. This allows us to pick up on potential
parse errors from the docs generator script as part of selftests.

The generated manual pages here are not intended for distribution, they
are just a fragment that can be integrated into the other static text of
bpf(2) to form the full manual page.

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-14-joe@cilium.io

Previously, the Makefile here was only targeting a single manual page so
it just hardcoded a bunch of individual rules to specifically handle
build, clean, install, uninstall for that particular page.

Upcoming commits will generate manual pages for an additional section,
so this commit prepares the makefile first by converting the existing
targets into an evaluated set of targets based on the manual page name
and section.

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-13-joe@cilium.io

This logic is used for validating the manual pages from selftests, so
move the infra under tools/testing/selftests/bpf/ and rely on selftests
for validation rather than tying it into the bpftool build.

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-12-joe@cilium.io