iperf: override bool detection

The configure script contains a buggy test for the bool datatype and
it's size. This is normally not a big deal as the result isn't used
for anything, but when BR2_CONFIG_CACHE (default) is used, the
wrong ac_cv_type_bool result is stored in the cache, and other packages
like libcurl DO need the bool type, so that breaks the build.

Fix it by presetting those tests to sane values.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>