mk: optimize directory dependencies

mk: optimize directory dependencies

Before this patch, the management of dependencies between directories
had several issues:

- the generation of .depdirs, done at configuration is slow: it can take
  more than one minute on some slow targets (usually ~10s on a standard
  PC without -j).

- for instance, it is possible to express a dependency like:
  - app/foo depends on lib/librte_foo
  - and lib/librte_foo depends on app/bar
  But this won't work because the directories are traversed with a
  depth-first algorithm, so we have to choose between doing 'app' before
  or after 'lib'.

- the script depdirs-rule.sh is too complex.

- we cannot use "make -d" for debug, because the output of make is used for
  the generation of .depdirs.

This patch moves the DEPDIRS-* variables in the upper Makefile, making
the dependencies much easier to calculate. A DEPDIRS variable is still
used to process library dependencies in LDLIBS.

After this commit, "make config" is almost immediate.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Tested-by: Robin Jarry <robin.jarry@6wind.com>
Tested-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>