Makefile: fix ldconfig selection for internal toolchains

Commit ed0d45fd (Choose host/target ldconfig based on availability)
added a runtime check for a cross-ldconfig being available.
Unfortunately this checks runs too early (at package/Makefile.in parsing
time), so it always fails when using an internal toolchain as ldconfig
isn't built yet.

Fix it by moving the check to the only place it is used (target-finalize).

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