gdb: enable target description support by default

This enables gdb target description support by default, it will check that
this is supported by the active target.

Change-Id: Iea5a5b94c5e373af30dacb780020b79536caa1e3
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1618


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>