core: move pkg-utils.mk to support/

core: move pkg-utils.mk to support/

pkg-utils.mk contains various definitions that are used in the package
infrastructures and packages themselves.

However, those definitions can be useful in other parts of Buildroot,
and are already used in a few places that are not related to the package
infrastructure. Also, $(sep) will be needed early in the Makefile when
we eventually support multiple br2-external trees.

Since this file only contains definitions, we can include it anytime.

So, consider that file to no longer be specific to the package infras:
  - move it to support and rename it,
  - move a few similar definitions from the main Makefile to that file.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>