buildroot

boost: make package less verbose when building with -s

The boost build is very verbose (11K+ lines even when no sub options are
enabled).  While this can be handy for debugging, make is less verbose when
'make -s' is used, similar to how we do it for autotools.

With this, make -s boost outputs less than 100 lines.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>