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>