Makefile: simplify target skeleton copying

We already use the .root stamp file to remember if we have copied the
skeleton, so we can remove the /bin check.

Likewise, we should always have a TARGET_SKELETON (or explicitly error
out otherwise), so remove that check as well.

Finally, now that we're using rsync to do the copy, we might as well
use its exclude support instead of cleaning up unwanted files afterwards.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>