openvpn: fix init script name

Our default (busybox) rc.S implementation requires init scripts to be
named S??*, so rename the openvpn one to S60openvpn (E.G. after network).

At the same time remove the deprecated check-if-custom-skeleton-provided-file
and just always install the init script. People can always fixup/remove
it in their post-build script if needed.

Also name the init script source the same as the destination file name in
TARGET_DIR for consistency.

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