tools: Add a help target

... and make it the default one so that calling 'make' without arguments
in the tools/ directory gives you the possible targets to build along
with a short description of what they are.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: http://lkml.kernel.org/r/1334162178-17152-5-git-send-email-bp@amd64.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>