cmake: move the host-pkgconf dependency from host-cmake to pkg-cmake

cmake: move the host-pkgconf dependency from host-cmake to pkg-cmake

In 3d475ee0ba4d6eea6aca25594cfe5bb153ac804f a dependency on
host-pkgconf was added to host-cmake. It is a workaround to fix build
failures for packages that use pkgconf through a cmake module, but do
not depend on host-pkgconf as they should.

Since it is the package that needs host-pkgconf and not host-cmake
itself, move the dependency to the proper place, in pkg-cmake.mk.

Also copy the explanation on the mentioned commit as a comment in
order to clarify why we do this.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Davide Viti <zinosat@tiscali.it>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas:
 - update on top of master
 - drop empty HOST_CMAKE_DEPENDENCIES, no longer needed since host
   dependencies are no longer derived from target dependencies.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>