Makefile: rename TARGETS to PACKAGES

For clarity, this commit renames the TARGETS variable to the more
meaningful PACKAGES variable. Indeed, only packages (handled by one of
the package infrastructures) should be listed in this variable, and
not other random non-package targets.

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