buildroot

Makefile: improve detection of make "-s" flag

Because it's just checking the presence of the "s" character even a
  make --warn-undefined-variables
is detected as a silent build.

Fix that by filtering out long options.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>