mk: do not generate LDLIBS from directory dependencies

mk: do not generate LDLIBS from directory dependencies

The list of libraries in LDLIBS was generated from the DEPDIRS-xyz
variable. This is valid when the subdirectory name match the library
name, but it's not always the case, especially for PMDs.

The patches removes this feature and explicitly adds the proper
libraries in LDLIBS.

Some DEPDIRS-xyz variables become useless, remove them.

Reported-by: Gage Eads <gage.eads@intel.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Reviewed-by: Gage Eads <gage.eads@intel.com>