Makefile: fix locale purge when BR2_PER_PACKAGE_DIRECTORIES=y

With BR2_PER_PACKAGE_DIRECTORIES=y, we have the following code in the
main Makefile:

target-finalize: $(PACKAGES) $(TARGET_DIR) host-finalize
	@$(call MESSAGE,"Finalizing target directory")
	$(call per-package-rsync,$(sort $(PACKAGES)),target,$(TARGET_DIR))
	$(foreach hook,$(TARGET_FINALIZE_HOOKS),$($(hook))$(sep))

The per-package-rsync call creates the global $(TARGET_DIR) from the
per-package $(TARGET_DIR). Then, we call the TARGET_FINALIZE_HOOKS.

One of the TARGET_FINALIZE_HOOKS, PURGE_LOCALES, remove locales that
are not desired by the user. It does so using a loop with the
$(wildcard ...) function.

However, the $(wildcard ...) function is expanded at the moment the
rule is evaluated. And with per-package directory, at the time the
rule is evaluated, the global $(TARGET_DIR) is empty, so $(wildcard
...) will return nothing. It is indeed only after the call to
per-package-rsync that the TARGET_DIR will be populated.

This commit fixes that by moving away from $(wildcard ...) and use a
shell test instead, since we are anyway in big block of shell code.

With this, locales are properly purged again when
BR2_PER_PACKAGE_DIRECTORIES=y.

Fixes: c4e6d5c8 ("core: implement per-package SDK and target")
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr:
  - make the style look like the code around (no space in front of ;)
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>