core: sort packages and eliminate duplicates in show-targets

core: sort packages and eliminate duplicates in show-targets

Currently, enabling more than one filesystem image will make
'show-targets' list a few host packages more than once.

This is because all filesystem images add the same set of
host-packages to their dependencies, which are then added as-is
to the package list.

Thus, host-fakeroot, host-makedevs and, if needed, host-mkpasswd will
appear as many times as there are filesystem images enabled.

Fix that by sorting the package list, thus eliminating duplicates from
that list. Also sort the rootfs list for good measure. Sort the two
separately, so that rootfses are last.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>