docs: update LIBFOO_SITE to describe use of filesystem paths

Buildroot supports the use of local filesystem paths in LIBFOO_SITE
since the 2011.11 release, but this is not documented in the manual.
Fix this and slightly reword the surrounding doucmentation
accordingly.

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>