build-sys: Move the print-variable rule to rules.mak

build-sys: Move the print-variable rule to rules.mak

Currently the print-variable rule can only be used in the
root directory:

  $ make print-vhost-user-json-y
  vhost-user-json-y= contrib/vhost-user-gpu/50-qemu-gpu.json tools/virtiofsd/50-qemu-virtiofsd.json

  $ make -C i386-softmmu print-obj-y
  make: Entering directory 'build/i386-softmmu'
  make: *** No rule to make target 'print-obj-y'.  Stop.
  make: Leaving directory 'build/i386-softmmu'

Move it to rules.mak so we can use it from other directories:

  $ make -C i386-softmmu print-obj-y
  make: Entering directory 'build/i386-softmmu'
  obj-y=qapi-introspect.o qapi-types-machine-target.o qapi-types-misc-target.o qapi-types.o qapi-visit-machine-target.o qapi-visit-misc-target.o qapi-visit.o qapi-events-machine-target.o qapi-events-misc-target.o qapi-events.o qapi-commands-machine-target.o qapi-commands-misc-target.o qapi-commands.o qapi-init-commands.o
  make: Leaving directory 'build/i386-softmmu'

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200306170456.21977-1-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>