cfg: add srst_nogate to the supported targets, remove from board configs

cfg: add srst_nogate to the supported targets, remove from board configs

It depends on the particular target whether it can work with SRST
asserted or not, so this belongs to the target config rather than the
board config.

Also, this allows for simple

openocd -f myboard.cfg -c "reset_config connect_assert_srst"

command to be used whenever a user feels a need to connect to an
unresponsive target.

Change-Id: I3d8da9ae47088fc0c75a20bfdd20074be1014de0
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2459
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>