Log Message:
Add buildlink/builtin mechanism for devel/editline and devel/readline.

With this change, .include "../../devel/readline/buildlink3.mk" with
USE_GNU_READLINE=yes should be replaced with
.include "../../devel/readline/buildlink3.mk",
and .include "../../devel/readline/buildlink3.mk" without USE_GNU_READLINE
should be replaced .include "../../mk/readline.buildlink3.mk".

USE_GNU_READLINE is removed.