pkg-download.mk: allow using localfiles outside of package infrastructure

The localfiles download method uses $($(PKG)_SITE))) and
$($(PKG)_SOURCE) instead of $(1) and $(2). This means that it can only
be used for package downloads (through gentargets, autotargets, ...)
and not for other downloads like external toolchains.

This patch changes localfiles to allow this, just as the wget and scp
download methods already did.
For the version control download methods, nothing changes.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>