dependencies.mk: check for valid host-tar before other host dependencies

host-{cmake,lzip,xz} needs host-tar to extract their source code tarball, so
we need to ensure that host-tar gets added to DEPENDENCIES_HOST_PREREQ
before these in case they are both needed, otherwise the tools will fail to
extract.

With the upcoming change to blacklist modern tar versions this situation is
likely to trigger more often.

The real solution to this issue is the <foo>_EXTRACT_DEPENDENCIES rework,
but that series is a bit too intrusive to add this close to 2018.02, so
therefore this hack.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>