dosfstools: make sure install targets aren't run everytime

Adding real targets to $(TARGETS) doesn't work, since they are
prefixed __real_tgt by the main Makefile. 'make' then always thinks
that these targets aren't up-to-date, and re-generate them every time.

Therefore, we switch dosfstools to the more common way of handling
packages, with a phony target added to $(TARGETS), this phony target
depending on real targets.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>