Update UNIFIED_OBJDIR descriptions

src.conf(5) previously stated they would be removed before FreeBSD 12.0,
but that did not happen. Change it to "a future version of FreeBSD."

Also pick up LOADER_KBOOT change (enabled on x86) in src.conf regen.

Reported by: jhb
MFC after: 3 days
Sponsored by: The FreeBSD Foundation