Makefile: directories are not PHONY targets

The global .PHONY specification in the main Makefile is describing a
number of directory targets as being PHONY, which doesn't make much
sense. PHONY targets are targets that do not exist on the filesystem,
and which make should always consider as not being up-to-date, so that
the commands associated to these targets are always executed by make
when the command is invoked, even if a file with the same name exists
on the filesystem.

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