dependencies: check core dependencies before anything else

Currently, the dependencies target (that runs dependencies.sh) depends on
DEPENDENCIES_HOST_PREREQ. This means that the dependencies listed in
DEPENDENCIES_HOST_PREREQ (currently host-sstrip if sstrip is selected) are
built *before* the dependencies.sh script is run.

As a result, if e.g. there is no gcc compiler present on the build system, the
dependencies in DEPENDENCIES_HOST_PREREQ will fail to build, and buildroot
will fail non-gracefully.

This patch makes sure that the DEPENDENCIES_HOST_PREREQ are checked *after* the
dependencies.sh script, so that any problem in the build system is reported in
a clean way by dependencies.sh.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>