target: fix init_targets script handling

This fixes an issue when init is called before init_targets has been
executed.

Make sure init_targets is called before init.

Change-Id: Icd5bd4c2a8eea2e399d9de4e331a77560e9672ac
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/235


Tested-by: jenkins
Reviewed-by: Peter Stuge <peter@stuge.se>