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

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 <[email protected]>
Cc: Arnout Vandecappelle <[email protected]>
Cc: Thomas De Schampheleire <[email protected]>
Cc: Thomas Petazzoni <[email protected]>
Cc: Peter Korsgaard <[email protected]>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <[email protected]>
Signed-off-by: Peter Korsgaard <[email protected]>