tools build: Allow setting the feature detection user

tools build: Allow setting the feature detection user

We will use the tools/build/ autodetection in the eBPF patchkit
and it is currently sharing the output directory with perf, that
also uses the feature detection logic.

As we keep state in the output directory, so that we can avoid running
all the tests again, we need to have different filenames for the files
used in this state, allow doing that via the FEATURE_USER variable, to
be set alongside the existing FEATURE_{TEST,DISPLAY} variables.

v2: Fix comment describing the FEATURE_DUMP filename to make sure where
    it is created, precisely at $(OUTPUT)FEATURE-DUMP$(FEATURE_USER).
    Pointed out by Jiri.

Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/n/tip-fdbev0vrn3x6idqc3ajbnvcb@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>