Add support for local packages through 'file://' pseudo-protocol

This can be used this way :

<pkg>_VERSION = 42
<pkg>_SITE = file:///some/local/directory


<pkg>_SOURCE = mypkg-$(<pkg>_VERSION).tar.bz2

Can be useful to integrate a home-made project or for testing purposes.
The default command to retrieve files is 'cp' but 'rsync' could also be used.
Through sshfs, it should also be possible to get non-public remote files on a
ssh server.

[ Thomas Petazzoni: use $(PKG)_SITE and $(PKG)_SOURCE variables
  instead of $(1) and $(2) ]
[ Peter: don't append $(QUIET), cp doesn't handle -q]
Signed-off-by: David Wagner <david.wagner@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>