boost.mk: allow building both static and shared

Use BR2_SHARED_STATIC_LIBS for building both static and shared boost libs.
The static libs are copied to the staging directory. Only the shared libs are
copied to the rootfs image.

[Thomas:
 - remove empty else clause, and instead add a comment that explains
   why we're doing something only for the BR2_STATIC_LIBS=y and
   BR2_SHARED_LIBS=y.
 - remove trailing backslash at the end of the BOOST_CONF_OPTS
   definition.]

Signed-off-by: Jonathan Ben Avraham <yba@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>