Move DPL_UNUSED & DPL_UNUSED_PARAM unused.h to availability.h #1

Move DPL_UNUSED & DPL_UNUSED_PARAM unused.h to availability.h #1

 * Task seperates two part
  1. Add DPL_UNUSED & DPL_UNUSED_PARAM to availability.h
  2. Remove unused.h after replace usage

[Issue#]       LINUXWRT-1016
[Problem]      Common header to support various macro for availability property is added
[Cause]        N/A
[Solution]     First part commit
               - Add DPL_UNUSED and DPL_UNUSED_PARAM to availability.h
               - All macro usage will replace from unused.h to availability.h
[Verification] Build all wrt packages
[SCMRequest]   N/A

Change-Id: I329e317e9733aaf52ab9b16473ea88ede4fdc59d