system: convert "system.mk" recipes to "target-finalize" hooks

Convert "system.mk" recipes to "target-finalize" hooks in order to:
- Ensure an ordering even if top-level parallel make is being used.
- Execute "system.mk" commands after the "target-finalize" initial message
  is printed so they can be clearly distinguished from packages
  building.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>