selftests/bpf: Exit with KSFT_SKIP if no Makefile found

This would happend when we run the tests after install kselftests
 root@lkp-skl-d01 ~# /kselftests/run_kselftest.sh -t bpf:test_doc_build.sh
 TAP version 13
 1..1
 # selftests: bpf: test_doc_build.sh
 perl: warning: Setting locale failed.
 perl: warning: Please check that your locale settings:
         LANGUAGE = (unset),
         LC_ALL = (unset),
         LC_ADDRESS = "en_US.UTF-8",
         LC_NAME = "en_US.UTF-8",
         LC_MONETARY = "en_US.UTF-8",
         LC_PAPER = "en_US.UTF-8",
         LC_IDENTIFICATION = "en_US.UTF-8",
         LC_TELEPHONE = "en_US.UTF-8",
         LC_MEASUREMENT = "en_US.UTF-8",
         LC_TIME = "en_US.UTF-8",
         LC_NUMERIC = "en_US.UTF-8",
         LANG = "en_US.UTF-8"
     are supported and installed on your system.
 perl: warning: Falling back to the standard locale ("C").
 # skip:    bpftool files not found!
 #
 ok 1 selftests: bpf: test_doc_build.sh # SKIP

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210820025549.28325-1-lizhijian@cn.fujitsu.com