dependencies: correct dependencies for top-level parallel build

dependencies: correct dependencies for top-level parallel build

Commit 4932c8a7cc94277b3d53cecd935f725b91cfa9fe introduced the
core-dependencies target to make sure that the dependencies.sh script
runs before we attempt to compile any host tool, so that the absence of
a compiler is properly detected. However, this relied on the
left-to-right evaluation of dependencies. This will no longer be true
when we enable top-level parallel build.

Fix this by letting DEPENDENCIES_HOST_PREREQ depend on
core-dependencies.

Note that it is not possible to remove the
dependencies <- core-dependencies. Indeed, it is possible that
DEPENDENCIES_HOST_PREREQ is completely empty, and in that case we still
need to check core-dependencies.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>