Allow nesting into directories without Build file. Currently we force
include of the Build file, which fails the build when the Build file is
missing.

We already support empty *-in.o' objects if there's nothing in the
directory to be compiled, so we can just use it for missing Build file
cases.

Also adding this case under tests.
Reported-by: Rabin Vincent <rabin.vincent@axis.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Rabin Vincent <rabin.vincent@axis.com>
Link: http://lkml.kernel.org/r/1432914178-24086-1-git-send-email-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>