Makefile: simplify show-targets

show-targets is only used currently by the graph-depends script, which
already recurses into the dependencies of the selected packages to
build the dependency graph. Therefore, dumping the contents of
$(PACKAGES) and $(ROOTFS_TARGETS) is sufficient: $(HOST_DEPS) and
$(TARGET_HOST_DEPS) will contain packages that are dependencies of
packages already listed in $(PACKAGES), which graph-depends will
discover by itself.

This allows to remove one more usage of $(HOST_DEPS) and
$(TARGET_HOST_DEPS), which is one more step towards their removal.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>