Makefile: remove unneeded variables

Now that all the external-deps, source-check and source targets are
properly implemented based on the package infrastructure, the
PACKAGES_SOURCE, TARGET_HOST_DEPS, HOST_DEPS and HOST_SOURCE variables
are no longer needed. This is a good thing since they were anyway
incorrect, as they were only doing a two level recursion in the
dependencies of host packages.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[tested with a randpackageconfig]