pkg-download: support 'file://' in BR2_PRIMARY_SITE

This feature was already discuted here:

  http://lists.busybox.net/pipermail/buildroot/2015-April/125419.html

Personally, I have a big central directory with all tarballs I have
ever downloaded. I use this feature to isolate tarballs necessary to
build a configuration:

  make project_defconfig
  make BR2_DL_DIR=/tmp/mirror-project BR2_PRIMARY_SITE=file:///home/user/dl source
  tar -C /tmp -czvf mirror-project.tgz mirror-project

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Config.in

@@ -165,8 +165,10 @@ config BR2_PRIMARY_SITE
 	  Primary site to download from. If this option is set then buildroot
 	  will try to download package source first from this site and try the
 	  default if the file is not found.
-	  Valid URIs are URIs recognized by $(WGET) and scp URIs of the form
-	  scp://[user@]host:path.
+	  Valid URIs are:
+	    - URIs recognized by $(WGET)
+	    - local URIs of the form file://absolutepath
+	    - scp URIs of the form scp://[user@]host:path.
 
 config BR2_PRIMARY_SITE_ONLY
 	bool "Only allow downloads from primary download site"

package/pkg-download.mk

@@ -214,6 +214,7 @@ endef
 define DOWNLOAD_INNER
 	$(Q)if test -n "$(call qstrip,$(BR2_PRIMARY_SITE))" ; then \
 		case "$(call geturischeme,$(BR2_PRIMARY_SITE))" in \
+			file) $(call $(3)_LOCALFILES,$(BR2_PRIMARY_SITE)/$(2),$(2)) && exit ;; \
 			scp) $(call $(3)_SCP,$(BR2_PRIMARY_SITE)/$(2),$(2)) && exit ;; \
 			*) $(call $(3)_WGET,$(BR2_PRIMARY_SITE)/$(2),$(2)) && exit ;; \
 		esac ; \