target/generic: add filesystem overlay option

The filesystem overlay is a tree that is copied over the target fs
after building everything - which is currently usually done in the
post-build script.

[Peter: don't ignore missing directories]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>