Only write release config files if they've changed

Currently, when you do `m` repeatedly, it does a little bit of
rebuilding to copy release config files around. If we change
release-config to only rewrite the files if they've changed, we
get a proper "ninja: no work to do." message.

Bug: 346757289
Test: m repeatedly
Change-Id: I9c1f6d34ec20d14b684a0183c5ec457ea92440f9