Deprecate jimtcl Git submodule

Deprecate jimtcl Git submodule

jimtcl was integrated as Git submodule for convenience and probably also
because packages were not widely available at the time. Today, jimtcl is
available in many popular package repositories [1] and the integration
as Git submodule adds unnecessary complexity to the OpenOCD build
process. For details, see the discussion on the mailing list in [2].

Disable the jimtcl Git submodule by default and announce it as
deprecated feature that will be removed in the next release. This gives
package maintainers time to adapt to the change and, if necessary,
build a package for jimtcl.

[1] https://repology.org/project/jimtcl/versions
[2] https://sourceforge.net/p/openocd/mailman/message/58786630/

Change-Id: I07930ac07f7d7a6317c08b21dc118f4f128b331c
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/8380
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>