at91sam7sx.cfg: fix use $_TARGETNAME as target identifier, not '0' warning

at91sam7sx.cfg: fix use $_TARGETNAME as target identifier, not '0' warning

all other at91 cfg files already has this fix.
It also fix "No flash at address 0x...." error when JTAG chain consist of
more than one at91sam7sx cores during attempt to flash other than first mcu
in chain.

Change-Id: I7785d9103d0fc494b6a823e2c73f850373ffe112
Signed-off-by: Sergey A. Borshch <sb-sf@users.sourceforge.net>
Reviewed-on: http://openocd.zylin.com/1812
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>