jimtcl: add configure flag for build maintainer mode

When jimtcl is built in maintainer mode, it runs extra tests at
exit to look for memory leak due to jim objects not properly freed
either through Jim_IncrRefCount()/Jim_DecrRefCount() or by passing
it to a jim API.

Add optional OpenOCD configure flag '--enable-jimtcl-maintainer'
to enable jimtcl maintainer mode.

Modify the implementation of macro AX_CONFIG_SUBDIR_OPTION to
allow expanding a variable passed as second argument.

Change-Id: Id1a39b25cee3773b172faf70803fa150182f0cd6
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6871
Tested-by: jenkins
Reviewed-by: zapb <dev@zapb.de>