Makefile: target-finalize: add TARGET_FINALIZE_HOOKS

Add TARGET_FINALIZE_HOOKS to the "target-finalize" rule to be able to
add to it commands as needed.
This is useful for having a nicer output because commands are executed
after the "target-finalize" initial message, also it is useful to ensure
an executing order even when top-level parallel makefile is being used.

Also convert "TARGET_PURGE_LOCALES" to a hook that uses
"TARGET_FINALIZE_HOOKS".

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>