pkg-infra: move the cp (aka localfiles) download helper to a script

Maintaining the download helpers in the Makefile has proved to be a bit
complex, so move it to a shell script.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
diff --git a/package/pkg-download.mk b/package/pkg-download.mk
index eec91b8..d3cd0c1 100644
--- a/package/pkg-download.mk
+++ b/package/pkg-download.mk
@@ -16,7 +16,7 @@
 export HG := $(call qstrip,$(BR2_HG)) $(QUIET)
 export SCP := $(call qstrip,$(BR2_SCP)) $(QUIET)
 SSH := $(call qstrip,$(BR2_SSH)) $(QUIET)
-LOCALFILES := $(call qstrip,$(BR2_LOCALFILES))
+export LOCALFILES := $(call qstrip,$(BR2_LOCALFILES))
 
 # Default spider mode is 'DOWNLOAD'. Other possible values are 'SOURCE_CHECK'
 # used by the _source-check target and 'SHOW_EXTERNAL_DEPS', used by the
@@ -192,7 +192,8 @@
 
 define DOWNLOAD_LOCALFILES
 	test -e $(DL_DIR)/$(2) || \
-		$(LOCALFILES) $(call stripurischeme,$(call qstrip,$(1))) $(DL_DIR)
+	$(EXTRA_ENV) support/download/cp $(call stripurischeme,$(call qstrip,$(1))) \
+					 $(DL_DIR)
 endef
 
 define SOURCE_CHECK_LOCALFILES