UBUNTU: SAUCE: selftests/bpf: clarify build error if no vmlinux

UBUNTU: SAUCE: selftests/bpf: clarify build error if no vmlinux

BugLink: https://bugs.launchpad.net/bugs/1908144
If Makefile cannot find any of the vmlinux's in its VMLINUX_BTF_PATHS list,
it tries to run btftool incorrectly, with VMLINUX_BTF unset:

    bpftool btf dump file $(VMLINUX_BTF) format c

Such that the keyword 'format' is misinterpreted as the path to vmlinux.
The resulting build error message is fairly cryptic:

      GEN      vmlinux.h
    Error: failed to load BTF from format: No such file or directory

This patch makes the failure reason clearer by yielding this instead:

    Makefile:...: *** cannot find a vmlinux for VMLINUX_BTF at any of
    "{paths}".  Stop.

Fixes: acbd06206bbb ("selftests/bpf: Add vmlinux.h selftest exercising tracing of syscalls")
Cc: stable@vger.kernel.org # 5.7+
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Reference: https://lore.kernel.org/bpf/20201214202049.7205-1-kamal@canonical.com/