* tools/Config.mk - Add empty definition POSTBUILD
* tools/Makefile.unix/win - Replace CXD56xx specific logic with $(call POSTBUILD)
* boards/arm/cxd56xx/scripts/Config.mk - Add POSTBUILD definitions with logic removed from Makefile.unix/win
* boards/arm/cxd56xx/spresense/scripts/Make.defs - Include the CXD56xx Config.mk immediately after tools/Config.mk so that the empty POSTBUILD definition will be replaced with the correct one
NOTE: There is a precedent for this approach. This is the way that other architecture-specific build options are implemented. See, for example, tools/zds/Config.mk
Gregory Nutt