build/sstrip: deprecate for 2014.05

Quoting Thomas on IRC:
  > sstrip is a piece of crap that we should exterminate
  > from the surface of the planet

sstrip is now unmaintained, has a long history of generating butchered
binaries in some cases, and does not gain that much space in the end.

We'd like to obliviate sstrip from the strip options, anihilate the
sstrip package, scorch any reference in fire, and cleanse our tree
free of any remnants...

But first, let's just deprecate it for now.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>