tools/bpftool: Force clean of out-of-tree build

mainline inclusion
from mainline-v5.11-rc1
commit 9e8929fd
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4S84A


CVE: NA

--------------------------------

Cleaning a partial build can fail if the output directory for libbpf
wasn't created:

$ make -C tools/bpf/bpftool O=/tmp/bpf clean
/bin/sh: line 0: cd: /tmp/bpf/libbpf/: No such file or directory
tools/scripts/Makefile.include:17: *** output directory "/tmp/bpf/libbpf/" does not exist.  Stop.
make: *** [Makefile:36: /tmp/bpf/libbpf/libbpf.a-clean] Error 2

As a result make never gets around to clearing the leftover objects. Add
the libbpf output directory as clean dependency to ensure clean always
succeeds (similarly to the "descend" macro). The directory is later
removed by the clean recipe.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201110164310.2600671-3-jean-philippe@linaro.org


Signed-off-by: Pu Lehui <pulehui@huawei.com>
Reviewed-by: Kuohai Xu <xukuohai@huawei.com>
Reviewed-by: Kuohai Xu <xukuohai@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>