scons: support out-of-tree builds

SCons has a slightly unusual implementation of out-of-tree builds where
there is no standard way of doing it. However, if a recipe sets B to
the idiomatic ${WORKDIR}/build and passes the right arguments so that this
is used then scons will fail as it is trying to find the SConstruct file
in ${B} not ${S}.

Always pass --directory=${S} to scons so that the SConstruct file can be
found, and don't call --clean if out-of-tree builds are being used as
we would have just deleted the entire build tree already.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>