boost: Pass staging dir to --with-icu option.

When using the --with-icu option without specifying the directory, boost's
bootstrap.sh script will look at "common" locations (lines 289-294):

    COMMON_ICU_PATHS="/usr /usr/local /sw"
    for p in $COMMON_ICU_PATHS; do
      if test -r $p/include/unicode/utypes.h; then
        ICU_ROOT=$p
      fi
    done

With buildroot it may surely become problematic at some point.

Signed-off-by: Ignacy Gawędzki <i@lri.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>