system: provide package-wide system variables and macros

system: provide package-wide system variables and macros

Some macros, soon some variables, currently defined in the skeleton are
going to be used by other packages.

Some of those variables will be used as Makefile conditions (e.g. in
ifeq() conditions), so they *must* be defined before being used.

Since the skeleton package, starting with an 's', is included quite
late, those variables would not be available to most packages.

Offload the existing macros into the new system/system.mk file, that is
included early, before any package is. Rename the macros to appropriate
names.

Future commits will add new macros and variables in that file.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>