Makefile: target-purgelocales: fix top-level parallel make support

The "target-purgelocales" target must be executed after all the other
targets and before the "target-finalize" target, so create a
TARGET_PURGE_LOCALES variable containing the commands of the target
"target-purgelocales" and add it at the beginning of the
"target-finalize" target.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>