bpftool: Remove redundant slashes

Because the OUTPUT variable ends with a slash but CURDIR doesn't, to keep
the _OUTPUT value consistent, we add a trailing slash to CURDIR when
defining _OUTPUT variable.

Since the _OUTPUT variable holds a value ending with a trailing slash,
there is no need to add another one when defining BOOTSTRAP_OUTPUT and
LIBBPF_OUTPUT variables. Likewise, when defining LIBBPF_INCLUDE and
LIBBPF_BOOTSTRAP_INCLUDE, we shouldn't add an extra slash either for the
same reason.

When building libbpf, the value of the DESTDIR argument should also not
end with a trailing slash.

Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220226163815.520133-1-ytcoode@gmail.com