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>

tcl/target/at91sam7sx.cfg

@@ -50,4 +50,4 @@ $_TARGETNAME configure -work-area-phys 0x00200000 -work-area-size 0x4000 -work-a
 
 #flash bank <driver> <base_addr> <size> <chip_width> <bus_width> <target_number> [<target_name> <banks> <sectors_per_bank> <pages_per_sector> <page_size> <num_nvmbits> <ext_freq_khz>]
 set _FLASHNAME $_CHIPNAME.flash
-flash bank $_FLASHNAME at91sam7 0 0 0 0 0 0 0 0 0 0 0 0 18432
+flash bank $_FLASHNAME at91sam7 0 0 0 0 $_TARGETNAME 0 0 0 0 0 0 0 18432