It fixes issues with the current rule that it is wrong that each
`$(BINDIR)/pkgname.a` is built by going in the package directory:

* `nordic_softdevice_ble.a` is built using `DIRS` and so the
  `$(APPLICATION_MODULE).a` target.
* It prevents having packages that are pseudomodules, which may be
  required to only use one "library" part of a package.

It also simplifies handling changes in 'ld -r' PR that could produce
objects instead of archives for packages.

Limitation of the current implementation
----------------------------------------

It removes rules being 'file' target and makes them depend on `.PHONY`
targets so always forces re-build.
But having a file target whose file is silently generated by another
target does not trigger a rebuild in Make.

They may have been declared as `order-only` prerequisites but as there are
some edge-cases that may not always work and does not currently add anything,
it was decided to keep them as normal prerequisites until it can be
globally solved.