testing: don't nest build for fp-test

testing: don't nest build for fp-test

Re-calling the main make is counter-productive and really messes up
with parallel builds. Just ensure we have built the pre-requisites
before we build the fp-test bits. If the user builds manually just
complain if the parent build hasn't got the bits we need.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Emilio G. Cota <cota@braap.org>